Spt5 modulates co-transcriptional spliceosome assembly in Saccharomyces cerevisiae

Abstract

There is increasing evidence from yeast to humans that pre-mRNA splicing occurs mainly co-transcriptionally, such that splicing and transcription are functionally coupled. Currently, there is little insight into the contribution of the core transcription elongation machinery to co-transcriptional spliceosome assembly and pre-mRNA splicing. Spt5 is a member of the core transcription elongation machinery and an essential protein, whose absence in budding yeast causes defects in pre-mRNA splicing. To determine how Spt5 affects pre-mRNA splicing, we used the auxin-inducible degron system to conditionally deplete Spt5 in Saccharomyces cerevisiae and assayed effects on co-transcriptional spliceosome assembly and splicing. We show that Spt5 is needed for efficient splicing and for the accumulation of U5 snRNPs at intron-containing genes, and therefore for stable co-transcriptional assembly of spliceosomes. The defect in co-transcriptional spliceosome assembly can explain the relatively mild splicing defect as being a consequence of the failure of co-transcriptional splicing. Co-immunoprecipitation of Spt5 with core spliceosomal proteins and all spliceosomal snRNAs suggests a model whereby Spt5 promotes co-transcriptional pre-mRNA splicing by stabilising the association of U5 snRNP with spliceosome complexes as they assemble on the nascent transcript. If this phenomenon is conserved in higher eukaryotes, it has potential to be important for co-transcriptional regulation of alternative splicing.