Construction of the eukaryotic ribosome is a complex process in which a nascent ribosomal RNA (rRNA) emerging from RNA Polymerase I hierarchically folds into a native three‐dimensional structure. Modular assembly of individual RNA domains through interactions with ribosomal proteins and a myriad of assembly factors permit efficient disentanglement of the error‐prone RNA folding process. Following these dynamic events, long‐range tertiary interactions are orchestrated to compact rRNA. A combination of genetic, biochemical, and structural studies is now providing clues into how (...) a nascent rRNA is transformed into a functional ribosome with high precision. With this essay, we aim to draw attention to the poorly understood process of establishing correct RNA tertiary contacts during ribosome formation. (shrink)

Ribosome biogenesis includes the making and processing of ribosomal RNAs, the biosynthesis of ribosomal proteins from their mRNAs in the cytosol and their transport to the nucleolus to assemble pre‐ribosomal particles. Several stresses including cellular senescence reduce nucleolar rRNA synthesis and maturation increasing the availability of ribosome‐free ribosomal proteins. Several ribosomal proteins can activate the p53 tumor suppressor pathway but cells without p53 can still arrest their proliferation in response to an imbalance between ribosomal proteins and mature rRNA (...) production. Recent results on senescence‐associated ribogenesis defects (SARD) show that the ribosomal protein S14 (RPS14 or uS11) can act as a CDK4/6 inhibitor linking ribosome biogenesis defects to the main engine of cell cycle progression. This work offers new insights into the regulation of the cell cycle and suggests novel avenues to design anticancer drugs. (shrink)

The conserved ribosomal protein uS3 in eukaryotes has long been known as one of the essential components of the small (40S) ribosomal subunit, which is involved in the structure of the 40S mRNA entry pore, ensuring the functioning of the 40S subunit during translation initiation. Besides, uS3, being outside the ribosome, is engaged in various cellular processes related to DNA repair, NF‐kB signaling pathway and regulation of apoptosis. This review is devoted to recent data opening new horizons in understanding (...) the roles of uS3 in such processes as the assembly and maturation of 40S subunits, ensuring proper structure of 48S pre‐initiation complexes, regulation of initiation and ribosome‐based RNA quality control pathways. Besides, we summarize novel results on the participation of the protein in processes beyond translation and consider biomedical implications of previously known and recently found extra‐ribosomal functions of uS3, primarily, in oncogenesis. (shrink)

The levels of different classes of mitochondrially encoded transcripts are developmentally regulated in sea urchin embryos, as a result of selection between mutually exclusive synthetic pathways. I propose a simple model to explain these observations, based on a dual role for mitochondrial ribosomes and translation factors in RNA synthesis as well as in translation. This effect may be exerted either at the transcriptional or post‐transcriptional level (or both), and is potentially generalizable to mammalian mtDNA and to other systems.

It was first suggested that the ribosome is associated with protein synthesis in the 1950s. Initially, its components were revealed as surface‐accessible proteins and as molecules of RNA apparently providing a scaffold for subunit shape. Attributing function to the proteins proved difficult, although bacterial protein L11 proved essential for binding one of the decoding protein release factors (RFs). With the discovery that RNA could be a catalyst, interest focussed on the rRNA that, in partnership with mRNA and tRNAs, could (...) potentially mediate the chemical reaction underlying protein synthesis. rRNA interactions and conformational changes were invoked as key elements that facilitated function. The decoding RFs, which are proteins, are exceptions to this rule because they usurp a tRNA function in mediating stop signal recognition. Cryoelectron microscopy and associated image reconstruction technology have now given dramatic snapshots of almost every step of protein synthesis, and X‐ray crystallography has revealed, at last, the subunits and monomeric ribosome in exquisite atomic detail. BioEssays 26:582–588, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

Autoantibodies to three eukaryotic 60S ribosomal phosphoproteins P0, P1 and P2 have been found in the sera of 10–20% of patients with systemic lupus erythematosus (SLE). These three proteins share a common epitope contained within the carboxy terminal 22 amino acids of each protein. Because central nervous system disturbances, with major behavioural disorders, occur in a significant fraction of SLE patients, the antiribosomal autoantibodies were measured in this subset of SLE individuals to determine whether or not there was an association. (...) This antibody is present in 90% of SLE patients who were diagnosed as having psychosis, secondary to the disease. (shrink)

Most eukaryotes possess many copies of rDNA. Organismal selection alone cannot maintain rRNA function because the effects of mutations in one rDNA are diluted by the presence of many other rDNAs. rRNA quality is maintained by processes that increase homogeneity of rRNA within, and heterogeneity among, germ cells thereby increasing the effectiveness of cellular selection on ribosomal function. A successful rDNA repeat will possess adaptations for spreading within tandem arrays by intranuclear selection. These adaptations reside in the non‐coding regions of (...) rDNA. Single‐copy genes are predicted to manage processes of intranuclear and cellular selection in the germline to maintain the quality of rRNA expressed in somatic cells of future generations. (shrink)

Eukaryotic genes are divided into three categories according to the machineries by which they are transcribed. Ribosomal RNA genes (rDNA) are the only ones that are transcribed by RNA polymerase I and are under different control from other genes transcribed by RNA polymerase II or III. None the less, the regulation of rDNA is of prime interest in view of its close relationship to cell growth and differentiation. In this review I shall discuss the recent progress in the study of (...) transcription mechanisms of rDNA by the RNA polymerase I system. The structure of the rDNA promoter and the presence of possible upstream enhancer regions will be described. In addition, factors involved in the transcription initiation of this gene will be discussed with special reference to the formation of an initiation complex. (shrink)

We discuss the role of linguistic metaphors as a cognitive frame for the understanding of genetic information processing. The essential similarity between language and genetic information processing has been recognized since the very beginning, and many prominent scholars have noted the possibility of considering genes and genomes as texts or languages. Most of the core terms in molecular biology are based on linguistic metaphors. The processing of genetic information is understood as some operations on text – writing, reading and editing (...) and their specification (encoding/decoding, proofreading, transcription, translation, reading frame). The concept of gene reading can be traced from the archaic idea of the equation of Life and Nature with the Book. Thus, the genetics itself can be metaphorically represented as some operations on text (deciphering, understanding, code-breaking, transcribing, editing, etc.), which are performed by scientists. At the same time linguistic metaphors portrayed gene entities also as having the ability of reading. In the case of such “bio-reading” some essential features similar to the processes of human reading can be revealed: this is an ability to identify the biochemical sequences based on their function in an abstract system and distinguish between type and its contextual tokens of the same type. Metaphors seem to be an effective instrument for representation, as they make possible a two-dimensional description: biochemical by its experimental empirical results and textual based on the cognitive models of comprehension. In addition to their heuristic value, linguistic metaphors are based on the essential characteristics of genetic information derived from its dual nature: biochemical by its substance, textual (or quasi-textual) by its formal organization. It can be concluded that linguistic metaphors denoting biochemical objects and processes seem to be a method of description and explanation of these heterogeneous properties. (shrink)

Applying mild methods of preparation, part of the ribosomes of rabbit reticulocytes are found in aggregates (later called polyribosomes) of up to six ribosomal units. Upon treatment with RNA-ase, they desintegrate into single ribosomes. The fast-sedimenting aggregates are found to be more active in protein synthesis in terms of incorporation of radioactive amino acids, whereas the single ribosomes are more receptive to stimulation by the artificial messenger RNA poly-U. The findings indicate that the linkage of ribosomes into aggregates is due (...) to the messenger RNA. They support a tape-reading mechanism of protein synthesis whereby growth of the peptide chain is accompanied by shifting the active site of the ribosome from one coding group of nucleotides of the messenger RNA to the next. (shrink)

We discuss the role of linguistic metaphors as a cognitive frame for the understanding of genetic information processing. The essential similarity between language and genetic information processing has been recognized since the very beginning, and many prominent scholars have noted the possibility of considering genes and genomes as texts or languages. Most of the core terms in molecular biology are based on linguistic metaphors. The processing of genetic information is understood as some operations on text – writing, reading and editing (...) and their specification (encoding/decoding, proofreading, transcription, translation, reading frame). The concept of gene reading can be traced from the archaic idea of the equation of Life and Nature with the Book. Thus, the genetics itself can be metaphorically represented as some operations on text (deciphering, understanding, code-breaking, transcribing, editing, etc.), which are performed by scientists. At the same time linguistic metaphors portrayed gene entities also as having the ability of reading. In the case of such “bio-reading” some essential features similar to the processes of human reading can be revealed: this is an ability to identify the biochemical sequences based on their function in an abstract system and distinguish between type and its contextual tokens of the same type. Metaphors seem to be an effective instrument for representation, as they make possible a two-dimensional description: biochemical by its experimental empirical results and textual based on the cognitive models of comprehension. In addition to their heuristic value, linguistic metaphors are based on the essential characteristics of genetic information derived from its dual nature: biochemical by its substance, textual (or quasi-textual) by its formal organization. It can be concluded that linguistic metaphors denoting biochemical objects and processes seem to be a method of description and explanation of these heterogeneous properties. (shrink)

Neurodegenerative syndromes present as proteinopathies – does ribosomal infidelity contribute to the protein toxicity that is the driving force for neuronal cell loss? Intracellular and extracellular protein aggregates overwhelm the clearance capacity of cells and tissues. Proteins aggregate when hydrophobic residues are exposed. Hydrophobic residues become exposed when proteins are misfolded. Protein misfolding can originate from translational errors at the ribosome. Indeed, the most error‐prone process in gene expression is translation at the ribosome. Recent evidence indicates that manipulating (...) the ribosomal accuracy impacts on the lifespan of model organisms and a reduced translational accuracy is accompanied by neurodegeneration. The first hit in aging‐associated neurodegenerative disease may be the well‐documented decline of cellular buffering capacity by aging. A second hit that impacts on the quality of protein synthesis could be the driving force for the observed loss of proteostasis in neurodegeneration. This hypothesis provides an explanation for the late onset of most neurodegenerative diseases. (shrink)

emerging field of nanotechnology. Nanotechnology gets its name from the nanometer, a distance of 10 -9 meters or roughly the diameter of a molecule, and the term refers to the technology for structuring matter with precise control at the nanometer scale, atom-by-atom or molecule-by-molecule, to form a pre-specified pattern. In other words, nanotechnology is the general ability to build large or small structures to complex atomic specifications.

Fundamental questions in evolution concern deep divisions in the living world and vertical versus horizontal information transfer. Two contrasting views are: (i) three superkingdoms Archaea, Eubacteria, and Eukarya based on vertical inheritance of genes encoding ribosomes; versus (ii) a prokaryotic/eukaryotic dichotomy with unconstrained horizontal gene transfer (HGT) among prokaryotes. Vertical inheritance implies continuity of cytoplasmic and structural information whereas HGT transfers only DNA. By hypothesis, HGT of the translation machinery is constrained by interaction between new ribosomal gene products and vertically (...) inherited cytoplasmic structure made largely of preexisting ribosomes. Ribosomes differentially enhance the assembly of new ribosomes made from closely related genes and inhibit the assembly of products from more distal genes. This hypothesis suggests experiments for synthetic biology: the ability of synthetic genomes to “boot,” i.e., establish hereditary continuity, will be constrained by the phylogenetic closeness of the cell “body” into which genomes are placed. (shrink)

The ribosomal polypeptide tunnel exit is the site where a variety of factors interact with newly synthesized proteins to guide them through the early steps of their biogenesis. In mitochondrial ribosomes, this site has been considerably modified in the course of evolution. In contrast to all other translation systems, mitochondrial ribosomes are responsible for the synthesis of only a few hydrophobic membrane proteins that are essential subunits of the mitochondrial respiratory chain. Membrane insertion of these proteins occurs co‐translationally and is (...) connected to a sophisticated assembly process that not only includes the assembly of the different subunits but also the acquisition of redox co‐factors. Here, we describe how mitochondrial translation is organized in the context of respiratory chain assembly and speculate how alteration of the ribosomal tunnel exit might allow the establishment of a subset of specialized ribosomes that individually organize the early steps in the biogenesis of distinct mitochondrially‐encoded proteins. (shrink)

We discuss the role of linguistic metaphors as a cognitive frame for the understanding of genetic information processing. The essential similarity between language and genetic information processing has been recognized since the very beginning, and many prominent scholars have noted the possibility of considering genes and genomes as texts or languages. Most of the core terms in molecular biology are based on linguistic metaphors. The processing of genetic information is understood as some operations on text – writing, reading and editing (...) and their specification (encoding/decoding, proofreading, transcription, translation, reading frame). The concept of gene reading can be traced from the archaic idea of the equation of Life and Nature with the Book. Thus, the genetics itself can be metaphorically represented as some operations on text (deciphering, understanding, codebreaking, transcribing, editing, etc.), which are performed by scientists. At the same time linguistic metaphors portrayed gene entities also as having the ability of reading. In the case of such “bio-reading” some essential features similar to the processes of human reading can be revealed: this is an ability to identify the biochemical sequences based on their function in an abstract system and distinguish between type and its contextual tokens of the same type. Metaphors seem to be an effective instrument for representation, as they make possible a two-dimensional description: biochemical by its experimental empirical results and textual based on the cognitive models of comprehension. In addition to their heuristic value, linguistic metaphors are based on the essential characteristics of genetic information derived from its dual nature: biochemical by its substance, textual (or quasi-textual) by its formal organization. It can be concluded that linguistic metaphors denoting biochemical objects and processes seem to be a method of description and explanation of these heterogeneous properties. (shrink)

We discuss the role of linguistic metaphors as a cognitive frame for the understanding of genetic information processing. The essential similarity between language and genetic information processing has been recognized since the very beginning, and many prominent scholars have noted the possibility of considering genes and genomes as texts or languages. Most of the core terms in molecular biology are based on linguistic metaphors. The processing of genetic information is understood as some operations on text – writing, reading and editing (...) and their specification (encoding/decoding, proofreading, transcription, translation, reading frame). The concept of gene reading can be traced from the archaic idea of the equation of Life and Nature with the Book. Thus, the genetics itself can be metaphorically represented as some operations on text (deciphering, understanding, codebreaking, transcribing, editing, etc.), which are performed by scientists. At the same time linguistic metaphors portrayed gene entities also as having the ability of reading. In the case of such “bio-reading” some essential features similar to the processes of human reading can be revealed: this is an ability to identify the biochemical sequences based on their function in an abstract system and distinguish between type and its contextual tokens of the same type. Metaphors seem to be an effective instrument for representation, as they make possible a two-dimensional description: biochemical by its experimental empirical results and textual according to the cognitive models of comprehension. In addition to their heuristic value, linguistic metaphors are based on the essential characteristics of genetic information derived from its dual nature: biochemical by its substance, textual (or quasitextual) by its formal organization. It can be concluded that linguistic metaphors denoting biochemical objects and processes seem to be a method of description and explanation of these heterogeneous properties. (shrink)

The translocation of tRNAs through the ribosome proceeds through numerous small steps in which tRNAs gradually shift their positions on the small and large ribosomal subunits. The most urgent questions are: (i) whether these intermediates are important; (ii) how the ribosomal translocase, the GTPase elongation factor G (EF‐G), promotes directed movement; and (iii) how the energy of GTP hydrolysis is coupled to movement. In the light of recent advances in biophysical and structural studies, we argue that intermediate states of (...) translocation are snapshots of dynamic fluctuations that guide the movement. In contrast to current models of stepwise translocation, kinetic evidence shows that the tRNAs move synchronously on the two ribosomal subunits in a rapid reaction orchestrated by EF‐G and GTP hydrolysis. EF‐G combines the energy regimes of a GTPase and a motor protein and facilitates tRNA movement by a combination of directed Brownian ratchet and power stroke mechanisms. (shrink)

Cryo‐electron microscopy allows the visualization of macromolecules in their native state. Combined with techniques of three‐dimensional reconstruction, cryo‐EM images of single molecules can be used to study macromolecular interactions. The ribosome, a large RNA–protein complex with multiple binding interactions, is an excellent test case illustrating the power of these new techniques. Conformational changes during the binding of tRNA and protein factors to the ribosome can now be studied without the interference of crystal packing. Now that the first X‐ray (...) structures of ribosomal subunits have become available, conformational changes observed by cryo‐EM in different functional states can be traced back to internal rearrangements of the underlying structural framework. Electron microscopy, X‐ray crystallography, and modeling should be used together in the endeavor to understand the functioning of the translational machinery. BioEssays 23:725–732, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

There are currently three major theories on the origin and evolution of the genetic code: the stereochemical theory, the coevolution theory, and the error-minimization theory. The first two assume that the genetic code originated respectively from chemical affinities and from metabolic relationships between codons and amino acids. The error-minimization theory maintains that in primitive systems the apparatus of protein synthesis was extremely prone to errors, and postulates that the genetic code evolved in order to minimize the deleterious effects of the (...) translation errors. This article describes a fourth theory which starts from the hypothesis that the ancestral genetic code was ambiguous and proposes that its evolution took place with a mechanism that systematically reduced its ambiguity and eventually removed it altogether. This proposal is distinct from the stereochemical and the coevolution theories because they do not contemplate any ambiguity in the genetic code, and it is distinct from the error-minimization theory because ambiguity-reduction is fundamentally different from error-minimization. The concept of ambiguity-reduction has been repeatedly mentioned in the scientific literature, but so far it has remained only an abstract possibility because no model has been proposed for its mechanism. Such a model is described in the present article and may be the first step in a new approach to the study of the evolution of the genetic code. (shrink)

The formation of mature large rRNAs from larger primary transcripts is very different in bacterial and mammalian cells. In both, cotranscription can help to assure the coordinated production of various rRNA species. However, in bacteria, processing is ordered, initiated by cleavages at double‐stranded stems which enclose the mature sequences; several cleavages are required to produce each mature terminus; and the final steps occur in polysomes, apparently linked to continued protein synthesis. In mouse cells, in contrast, cleavages generate nearly all mature (...) termini stochastically and directly, with no evidence for double‐stranded stems as cleavage signals; and processing occurs in the nucleolus, long before any new rRNA chains arrive in polysomes. Spacer sequences also serve different potnetial functions at the evolutionary extremes. In bacteria, transcribed spacer sequences are associated with different secondary structures in the pre‐rRNA compared to the mature rRNA, and these may promote ribosome formation. In mammalian cells, analyses are too premature to assign any comparable function to transcribed spacer sequences, but the non‐transcribed spacers are probably critical in the organization and replication of nucleoli, functions that are absent from bacteria. (shrink)

Translation of the genetic code occurs in a cycle where ribosomes engage mRNAs, synthesize protein, and then disengage in order to repeat the process again. The final part of this process—ribosome recycling, where ribosomes dissociate from mRNAs—involves a complex molecular choreography of specific protein factors to remove the large and small subunits of the ribosome in a coordinated fashion. Errors in this process can lead to the accumulation of ribosomes at stop codons or translation of downstream open reading (...) frames (ORFs). Ribosome recycling is also critical when a ribosome stalls during the elongation phase of translation and must be rescued to allow continued translation of the mRNA. Here we discuss the molecular interactions that drive ribosome recycling, and their regulation in the cell. We also examine the consequences of inefficient recycling with regards to disease, and its functional roles in synthesis of novel peptides, regulation of gene expression, and control of mRNA‐associated proteins. Alterations in ribosome recycling efficiency have the potential to impact many cellular functions but additional work is needed to understand how this regulatory power is utilized. (shrink)

It has been known for 20 years that the ribosomal protein S6 is rapidly phosphorylated when cells are stimulated to grow or divide. Furthermore, numerous studies have documented that there is a strong correlation between increases in S6 phosphorylation and protein synthesis, leading to the idea that S6 phosphorylation is involved in up‐regulating translation. In an attempt to define a mechanism by which S6 phosphorylation exerts translational control, other studies have focused on characterizing the sites of phosphorylation of this protein (...) and its location within the ribosome. Recent data show that S6 is a protein which may have diverse cellular functions and is essential for normal development, and that it may be involved in the translational regulation of a specific class of messages. (shrink)

Life appears to be a natural property of matter, but the problem of its origin only arose after early scientists refuted continuous spontaneous generation. There is no chance of life arising ‘all at once’, we need the standard scientific incremental explanation with large numbers of small steps, an approach used in both physical and evolutionary sciences. The necessity for considering both theoretical and experimental approaches is emphasized. After describing basic principles that are available (including the Darwin-Eigen cycle), the search for (...) origins is considered under four main themes. These are the RNA-world hypothesis; potential intermediates between an RNA-world and a modern world via the evolution of protein synthesis and then of DNA; possible alternatives to an RNA-world; and finally the earliest stages from the simple prebiotic systems to RNA. The triplicase/proto-ribosome theory for the origin of the ribosome is discussed where triples of nucleotides are added to a replicating RNA, with the origin of a triplet code well-before protein synthesis begins. The length of the code is suggested to arise from the early development of a ratchet mechanism that overcomes the problem of continued processivity of an RNA-based RNA-polymerase. It is probable that there were precursor stages to RNA with simpler sugars, or just two nucleotides, but we do not yet know of any better alternatives to RNA that were likely to arise naturally. For prebiotic stages (before RNA) a flow-reactor model is suggested to solve metabolism, energy gradients, and compartmentation simultaneously – thus the intense interest in some form of flow reactor. If an autocatalytic cycle could arise in such a system we would be major steps ahead. The most likely physical conditions for the origin of life require further clarification and it is still unclear whether the origin of life is more of an entropy (information) problem (and therefore high temperatures would be detrimental), rather than a kinetic problem (where high temperatures may be advantageous). (shrink)

The cellular internal ribosomal entry site (IRES) is one of the most important elements to mediate cap‐independent translational initiation, especially under conditions of stress and pathology. However, a high‐throughput method to discover IRESs in these conditions is still lacking. Here, a possible way IRES long‐read sequencing based on the latest high‐throughput technologies is proposed to solve this problem. Based on this design, diversity and integrity of the transcriptome from original samples can be kept. The micro‐environment that stimulates or inhibits IRES (...) activity can also be mimicked. By using long read‐length sequencing technology, additional experiments that are essential for ruling out the cryptic promoters or splicing events in routine IRES identification processes can be circumvented. It is hoped that this proposed methodology may be adopted for IRES element discovery, hence uncovering the full extent of the role of IRESs in disease, development, and stress. Also see the video abstract here https://youtu.be/JuWBbMzWXS8. (shrink)

Mitochondria are increasingly being recognized as information hubs that sense cellular changes and transmit messages to other cellular components, such as the nucleus, the endoplasmic reticulum (ER), the Golgi apparatus, and lysosomes. Nonetheless, the interaction between mitochondria and the nucleus is of special interest because they both host part of the cellular genome. Thus, the communication between genome‐bearing organelles would likely include gene expression regulation. Multiple nuclear‐encoded proteins have been known to regulate mitochondrial gene expression. On the contrary, no mitochondrial‐encoded (...) factors are known to actively regulate nuclear gene expression. MOTS‐c (mitochondrial open reading frame of the 12S ribosomal RNA type‐c) is a recently identified peptide encoded within the mitochondrial 12S ribosomal RNA gene that has metabolic functions. Notably, MOTS‐c can translocate to the nucleus upon metabolic stress (e.g., glucose restriction and oxidative stress) and directly regulate adaptive nuclear gene expression to promote cellular homeostasis. It is hypothesized that cellular fitness requires the coevolved mitonuclear genomes to coordinate adaptive responses using gene‐encoded factors that cross‐regulate the opposite genome. This suggests that cellular gene expression requires the bipartite split genomes to operate as a unified system, rather than the nucleus being the sole master regulator. (shrink)

Graphical AbstractYeast mitochondria frequently mutate, and some dysfunctional mitochondria out-compete wild-type versions. The retrograde response enables yeast to tolerate dysfunction, but also produces ribosomal DNA circles (ERCs). We propose that ERC accumulation increases expression of the rDNA antisense gene, TAR1, which counteracts spread of respiration-deficient mitochondria in matings with wild-type yeast.

The coupling of protein synthesis and folding is a crucial yet poorly understood aspect of cellular protein folding. Over the past few years, it has become possible to experimentally follow and define protein folding on the ribosome, revealing principles that shape co‐translational folding and distinguish it from refolding in solution. Here, we highlight some of these recent findings from biochemical and biophysical studies and their potential significance for cellular protein biogenesis. In particular, we focus on nascent chain interactions with (...) the ribosome, interactions within the nascent protein, modulation of translation elongation rates, and the role of mechanical force that accompanies nascent protein folding. The ability to obtain mechanistic insight in molecular detail has set the stage for exploring the intricate process of nascent protein folding. We believe that the aspects discussed here will be generally important for understanding how protein synthesis and folding are coupled and regulated. (shrink)

Over the last decade, our appreciation of the importance of the nucleolus for cellular function has progressed from the ordinary to the extraordinary. We no longer think of the nucleolus as simply the site of ribosome production, or a dynamic subnuclear body noted by pathologists for its changes in size and shape with malignancy. Instead, the nucleolus has emerged as a key controller of many cellular processes that are fundamental to normal cell homeostasis and the target for dysregulation in (...) many human diseases; in some cases, independent of its functions in ribosome biogenesis. These extra‐nucleolar or new functions, which we term “non‐canonical” to distinguish them from the more traditional role of the nucleolus in ribosome synthesis, are the focus of this review. In particular, we explore how these non‐canonical functions may provide novel insights into human disease and in some cases new targets for therapeutic development. (shrink)

Complexes of two or more proteins form many, if not most, of the intracellular “machines” that execute physical and chemical work, and transmit information. Complexes can form from stochastic post‐translational interactions of fully formed proteins, but recent attention has shifted to co‐translational interactions in which the most common mechanism involves binding of a mature constituent to an incomplete polypeptide emerging from a translating ribosome. Studies in yeast have revealed co‐translational interactions during formation of multiple major complexes, and together with (...) recent mammalian cell studies, suggest widespread utilization of the mechanism. These translation‐dependent interactions can involve a single or multiple mRNA templates, can be uni‐ or bi‐directional, and can use multi‐protein sub‐complexes as a binding component. Here, we discuss benefits of co‐translational complex assembly including accuracy and efficiency, overcoming hidden interfaces, localized and hierarchical assembly, and reduction of orphan protein degradation, toxicity, and dominant‐negative pathogenesis, all serving to improve cell fitness. (shrink)

The egg behaves as a prospective cell sustaining the developmental processes of the future embryo. In biosemiotic terms, this apparent teleonomic behaviour can be accounted for without referring to the exclusive causal role played by its genetic makeup. We envision two different processes that are uniquely found in the oocyte: (1) the first involves the mechanisms by which large amounts of mRNA accumulate in the ooplasm to establish the embryo axes prior to fertilization; (2) the second involves transfer of an (...) excess of maternally supplied ribosomes to the oocyte to provide the future embryo with newly synthesized proteins. In this paper, we argue that the information required to sustain embryonic development is not due to any physical properties of the zygotic DNA and the maternal mRNAs, but to their spatially and temporally ordered relationship in the zygote’s internal space. (shrink)

Bacteria use trans‐translation to rescue stalled ribosomes and target incomplete proteins for proteolysis. Despite similarities between tRNAs and transfer‐messenger RNA (tmRNA), the key molecule for trans‐translation, new structural and biochemical data show important differences between translation and trans‐translation at most steps of the pathways. tmRNA and its binding partner, SmpB, bind in the A site of the ribosome but do not trigger the same movements of nucleotides in the rRNA that are required for codon recognition by tRNA. tmRNA‐SmpB moves (...) from the A site to the P site of the ribosome without subunit rotation to generate hybrid states, and moves from the P site to a site outside the ribosome instead of to the E site. During catalysis, transpeptidation to tmRNA appears to require the ribosomal protein bL27, which is dispensable for translation, suggesting that this protein may be conserved in bacteria due to trans‐translation. These differences provide insights into the fundamental nature of trans‐translation, and provide targets for new antibiotics that may have decrease cross‐reactivity with eukaryotic ribosomes. (shrink)

Ribosomal RNA transcription was one of the first model systems for molecular characterization of a transcription regulatory mechanism and certainly one of the best studied in the widest range of organisms. In multicellular organisms, however, the issue of cell‐type‐specific regulation of rRNA transcription has not been well addressed. Here I propose that a systematic study of cell‐type‐specific regulation of rRNA transcription may reveal new regulatory mechanisms that have not been previously realized. Specifically, issues concerning the cell‐type‐specific requirement for rRNA production, (...) the universality of Pol I transcription complex and the division of rDNA into regulatory subdomains are discussed. BioEssays 28: 719–725, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Electron micrographs of active ribosomal genes from many species show a similar picture in which gene regions covered with nascent transcripts alternate with apparently non‐transcribed spacers. Since the gradients of visible nascent transcripts stop near the 3′ end of the 28S sequence it has often been assumed that transcription by RNA polymerase I also terminates at that point. Recent biochemical studies have shown however, that transcription continues far beyond the 3′ end of the 28S and in some species continues across (...) the entire spacer. In this article we review the evidence for spacer transcription in Xenopus, mouse and Drosophila and discuss possible reasons for its invisibility in the electron microscope. (shrink)

Modern cells present no signs of a putative prebiotic RNA world. However, RNA coding is not a sine qua non for the accumulation of catalytic polypeptides. Thus, cellular proteins spontaneously fold into active structures that are resistant to proteolysis. The law of mass action suggests that binding domains are stabilized by specific interactions with their substrates. Random polypeptide synthesis in a prebiotic world has the potential to initially produce only a very small fraction of polypeptides that can fold spontaneously into (...) catalytic domains. However, that fraction can be enriched by proteolytic activities that destroy the unfolded polypeptides and regenerate amino acids that can be recycled into polypeptides. In this open system scenario the stable domains that accumulate and the chemical environment in which they are accumulated are linked through self coding of polypeptide structure. Such open polypeptide systems may have been the precursors to the cellular ribonucleoprotein (RNP) world that evolved subsequently. (shrink)

Graphical AbstractCellular internal ribosomal entry site (IRES) is important for cap-independent translational initiation during stress and pathology. In article number 1900180, Lei-Yun Wang et al. propose an idea to discover IRES based on the latest high-throughput technologies, hence facilitating the understanding of IRES in disease development and treatment.

The origin of the bilaterian metazoans from radial ancestors is one of the biggest puzzles in animal evolution. A way to solve it is to identify the nature and main features of the last common ancestor of the bilaterians (LCB). Recent progress in molecular phylogeny has shown that many platyhelminth flatworms, regarded for a long time as basal bilaterians, now belong to the lophotrochozoan protostomates. In contrast, the LCB is now considered a complex organism bearing several features of modern bilaterians. (...) Here we discuss an alternative view, in which acoelomorph (Acoela + Nemertodermatida) flatworms, which do not belong to the Platyhelminthes, represent the earliest extant bilaterian clade. Sequences from ribosomal and other nuclear genes, Hox cluster genes, and reinterpretation of some morphological features strongly support the basal position of acoelomorphs arguing against a complex LCB. This reconstruction backs the old planuloid–acoeloid hypothesis and may help our understanding of the evolution of body axes, Hox genes and the Cambrian explosion. BioEssays 26:1046–1057, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

It should now be recognized that codes are central to life and to understanding its more complex forms, including human culture. Recognizing the ‘conventional’ nature of codes provides solid grounds for rejecting efforts to reduce life to biochemistry and justifies according a place to semantics in life. The question I want to consider is whether this is enough. Focussing on Eigen’s paradox of how a complex code could originate, I will argue that along with Barbieri’s efforts to account for the (...) origins of life based on the ribosome and then to account for the refined codes through a process of ambiguity reduction, something more is required. Barbieri has not provided an adequate account of emergence, or the basis for providing such an account. I will argue that Stanley Salthe has clarified to some extent the nature of emergence by conceptualizing it as the interpolation of new enabling constraints. Clearly, codes can be seen as enabling constraints. How this actually happens, though, is still not explained. Stuart Kauffman has grappled with this issue and shown that it radically challenges the assumptions of mainstream science going back to Newton. He has attempted to reintroduce real possibilities or potentialities into his ontology, and argued that radically new developments in nature are associated with realizing adjacent possibles. This is still not adequate. What is also involved, I will suggest, utilizing concepts developed by the French natural philosopher Gilbert Simondon, is ‘transduction’ as part of ‘ontogenesis’ of individuals in a process of ‘individuation’, that is, the emergence of ‘individuals’ from preindividual fields or milieux. (shrink)

Several features of the yeast mitochondrial genome, including high mutation rate, dynamic genomic structure, small effective population size, and dispensability for cellular viability, make it a promising candidate for generating hybrid incompatibility and driving speciation. Cytonuclear incompatibility, a specific type of Dobzhansky‐Muller genetic incompatibility caused by improper interactions between mitochondrial and nuclear genomes, has previously been observed in a variety of organisms, yet its role in speciation remains obscure. Recent studies in Saccharomyces yeast species provide a new insight, with experimental (...) evidence that cytonuclear incompatibility and DNA sequence divergence are both causes of the reproductive isolation of different yeast species. Interestingly, these two mechanisms seem to be perfectly complementary to each other in terms of their effects and evolutionary trajectories. Direct molecular analyses of the incompatible genes in yeasts have started to shed light on the evolutionary forces driving speciation. Editor's suggested further reading in BioEssaysThe cytoplasmic structure hypothesis for ribosome assembly, vertical inheritance, and phylogeny AbstractMitochondrial bioenergetics as a major motive force of speciation Abstract. (shrink)

Many apparently complex mechanisms in biology, especially in embryology and molecular biology, can be explained easily by reasoning at the level of the “efficient cause” of the observed phenomenology: the mechanism can then be explained by a simple geometrical argument or a variational principle, leading to the solution of an optimization problem, for example, via the co-existence of a minimization and a maximization problem . Passing from a microscopic level to the macroscopic level often involves an averaging effect that gives (...) birth to a global functional feature . We will illustrate these general principles by building in four different domains of application “a minima” models and showing the main properties of their solutions: extraction of a minimal RNA structure functioning as the first “peptidic machine,” a kind of ancestral ribosome; study of a genetic regulatory network of Drosophila centred on Engrailed gene and expressing successively two genes inside a limit cycle; study of a genetic network regulating neural activity and proliferation in mammals; and study of a simple geometric model of epiboly in zebrafish. (shrink)

tRNAs undergo multiple conformational changes during the translation cycle that are required for tRNA translocation and proper communication between the ribosome and translation factors. Recent structural data on how destabilized tRNAs utilize the CCA‐adding enzyme to proofread themselves put a spotlight on tRNA flexibility beyond the translation cycle. In analogy to tRNA surveillance, this review finds that other processes also exploit versatile tRNA folding to achieve, amongst others, specific aminoacylation, translational regulation by riboswitches or a block of bacterial translation. (...) tRNA flexibility is thereby not restricted to the hinges utilized during translation. In contrast, the flexibility of tRNA is distributed all over its L‐shape and is actively exploited by the tRNA‐interacting partners to discriminate one tRNA from another. Since the majority of tRNA modifications also modulate tRNA flexibility it seems that cells devote enormous resources to tightly sense and regulate tRNA structure. This is likely required for error‐free protein synthesis. (shrink)

RNA modifications have recently emerged as an important regulatory layer of gene expression. The most prevalent and reversible modification on messenger RNA (mRNA), N6‐methyladenosine, regulates most steps of RNA metabolism and its dysregulation has been associated with numerous diseases. Other modifications such as 5‐methylcytosine and N1‐methyladenosine have also been detected on mRNA but their abundance is lower and still debated. Adenosine to inosine RNA editing is widespread on coding and non‐coding RNA and can alter mRNA decoding as well as protect (...) against autoimmune diseases. 2′‐O‐methylation of the ribose and pseudouridine are widespread on ribosomal and transfer RNA and contribute to proper RNA folding and stability. While the understanding of the individual role of RNA modifications has now reached an unprecedented stage, still little is known about their interplay in the control of gene expression. In this review we discuss the examples where such interplay has been observed and speculate that with the progress of mapping technologies more of those will rapidly accumulate. (shrink)

Prokaryotes are generally assumed to be the oldest existing form of life on earth. This assumption, however, makes it difficult to understand certain aspects of the transition from earlier stages in the origin of life to more complex ones, and it does not account for many apparently ancient features in the eukaryotes. From a model of the RNA world, based on relic RNA species in modern organisms, one can infer that there was an absolute requirement for a high-accuracy RNA replicase (...) even before proteins evolved. In addition, we argue here that the ribosome (together with the RNAs involved in its assembly) is so large that it must have had a prior function before protein synthesis. A model that connects and equates these two requirements (high-accuracy RNA replicase and prior function of the ribosome) can explain many steps in the origin of life while accounting for the observation that eukaryotes have retained more vestiges of the RNA world. The later derivation of prokaryote RNA metabolism and genome structure can be accounted for by the two complementary mechanisms of r-selection and thermoreduction. BioEssays 21:880–889, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Metagenomics bears upon all aspects of microbiology, including our understanding of mitochondrial and eukaryote origin. Recently, ribosomal protein phylogenies show the eukaryote host lineage – the archaeal lineage that acquired the mitochondrion – to branch within the archaea. Metagenomic studies are now uncovering new archaeal lineages that branch more closely to the host than any cultivated archaea do. But how do they grow? Carbon and energy metabolism as pieced together from metagenome assemblies of these new archaeal lineages, such as the (...) Deep Sea Archaeal Group (including Lokiarchaeota) and Bathyarchaeota, do not match the physiology of any cultivated microbes. Understanding how these new lineages live in their environment is important, and might hold clues about how mitochondria arose and how the eukaryotic lineage got started. Here we look at these exciting new metagenomic studies, what they say about archaeal physiology in modern environments, how they impact views on host‐mitochondrion physiological interactions at eukaryote origin. (shrink)

The theory of chemical symbiosis suggests that biological systems started with the collaboration of two polymeric molecules existing in early Earth: nucleic acids and peptides. Chemical symbiosis emerged when RNA-like nucleic acid polymers happened to fold into 3D structures capable to bind amino acids together, forming a proto peptidyl-transferase center. This folding catalyzed the formation of quasi-random small peptides, some of them capable to bind this ribozyme structure back and starting to form an initial layer that would produce the larger (...) subunit of the ribosome by accretion. TCS suggests that there is no chicken-and-egg problem into the emergence of biological systems as RNAs and peptides were of equal importance to the origin of life. Life has initially emerged when these two macromolecules started to interact in molecular symbiosis. Further, we suggest that life evolved into progenotes and cells due to the emergence of new layers of symbiosis. Mutualism is the strongest force in biology, capable to create novelties by emergent principles; on which the whole is bigger than the sum of the parts. TCS aims to apply the Margulian view of biology into the origins of life field. (shrink)

Pre‐existing but untranslated or ‘poised’ mRNA exists as a means to rapidly induce the production of specific proteins in response to stimuli and as a safeguard to limit the actions of these proteins. The translation of poised mRNA enables immune cells to express quickly genes that enhance immune responses. The molecular mechanisms that repress the translation of poised mRNA and, upon stimulation, enable translation have yet to be elucidated. They likely reflect intrinsic properties of the mRNAs and their interactions with (...) trans‐acting factors that direct poised mRNAs away from or into the ribosome. Here, I discuss mechanisms by which this might be regulated. (shrink)

The very high genome copy number in cytoplasmic organelles is a puzzling fact in cell biology. It is proposed here that high copy number reflects an increased need for organellar ribosomes that can only be satisfied by the increased ribosomal RNA gene number that results from genome multiplication.