FIGURE 6.

Model for efficient removal of multiple introns. Splice sites are recognized during transcription and spliceosomal components initially associate with the pre-mRNA in exon definition complexes (stretched oval over each exon). A transition to cross-intron interactions occurs during splice site pairing (gray circles) and components previously involved in exon definition interactions remain associated with the pre-mRNA following intron removal (white circles). An EJC (black circle) is deposited during intron removal, but has no measurable effect on the splicing of neighboring introns. However, snRNP retention at the splice sites promotes efficient neighboring intron removal, because ligated exons do not require de novo splice site recognition. The cotranscriptional nature of initial splice site recognition is indicated by the presence of Pol II at the 3′ end of the pre-mRNA.