Uncoupling yeast intron recognition from transcription with recursive splicing.

Pre-mRNA splicing has to be coordinated with other processes occurring in the nucleus including transcription, mRNA 3' end formation and mRNA export. To analyze the relationship between transcription and splicing, we constructed a network of nested introns. Introns were inserted in the 5' splice site and/or branchpoint of a synthetic yeast intron interrupting a reporter gene. The inserted introns mask the recipient intron from the cellular machinery until they are removed by splicing. Production of functional mRNA from these constructs therefore requires recognition of a spliced RNA as a splicing substrate. We show that recurrent splicing occurs in a sequential and ordered fashion in vivo. Thus, in Saccharomyces cerevisiae, intron recognition and pre-spliceosome assembly is not tightly coupled to transcription.