Abstract

Enormous phylogenetic variation exists in the number and sizes of introns in protein‐coding genes. Although some consideration has been given to the underlying role of the population‐genetic environment in defining such patterns, the influence of the intracellular environment remains virtually unexplored. Drawing from observations on interactions between co‐transcriptional processes involved in splicing and mRNA 3′‐end formation, a mechanistic model is proposed for splice‐site recognition that challenges the commonly accepted intron‐ and exon‐definition models. Under the suggested model, splicing factors that outcompete 3′‐end processing factors for access to intronic binding sites concurrently favor the recruitment of 3′‐end processing factors at the pre‐mRNA tail. This hypothesis sheds new light on observations such as the intron‐mediated enhancement of gene expression and the negative correlation between intron length and levels of gene expression.