13 Splicing of Precursors to mRNA by the Spliceosome

Abstract

I. INTRODUCTION
The RNA world pictures a time when RNA structures catalyzed many reactions, almost certainly including RNA splicing. The self-splicing reactions of group I and group II introns may be direct remnants of the RNA world. Both of these catalytic structures can facilitate cis-splicing, where the two exons are initially part of a contiguous RNA, and trans-splicing, where the two exons are initially on two separate RNAs. In the latter case, the partial intron sequences flanking the two exons pair through sequence complementarity to form a catalytic structure joining the associated exons.

Many have speculated that the exon-intron structure was common in the genetic material in the RNA world. In fact, exons have been hypothesized as having originally evolved as units of sequences which encoded functional protein domains that were assembled by RNA splicing into large multidomain proteins with more complex functions. In this period, it is highly likely that RNA splicing was the product of the action of group I- and group II-like catalytic structures, and peptide-bond formation was catalyzed by progenitors of ribosomal RNA. As the potential to generate specific proteins evolved, some of these proteins probably bound to sequences within the catalytic introns to facilitate the rate of splicing. Eventually, the situation must have evolved to a stage where a fragment of RNA derived from a catalytic group I- or group II-like intron was transcribed in abundance and began to associate in trans with many different self-splicing introns to facilitate splicing. Perhaps the RNA fragment promoted...