UC San Diego

In the last several years, a number of studies have shown that spliceosome assembly and splicing catalysis can occur co-transcriptionally. However, it has been unclear which specific transcription factors play key roles in coupling splicing to transcription and the mechanisms through which they act. Here we report the discovery that Gcn5, which encodes the histone acetyltransferase (HAT) activity of the SAGA complex, has HAT-dependent genetic interactions with the genes encoding the heterodimeric U2 snRNP proteins Msl1 and Lea1, suggesting a functional relationship between Gcn5 HAT activity and Msl1/Lea1 function. To understand this relationship, we carried out an analysis of Gcn5's role in co-transcriptional recruitment of Msl1 and Lea1 to pre-mRNA and find that Gcn5 HAT activity is required for co-transcriptional recruitment of the U2 snRNP (and subsequent snRNP) components to the branchpoint. Although previous studies suggested that transcription elongation can alter co- transcriptional pre-mRNA splicing, we do not observe evidence of defective transcription elongation for these genes in the absence of Gcn5, while Gcn5-dependent histone acetylation is enriched in the promoter regions. While all these data suggest a role for histone acetylation in co- transcriptional spliceosome assembly. A closer examination of the functional interactions between histone mutants and the U2 snRNP and the effects of histone mutants and histone deacetylation on spliceosome assembly provide convincing evidence of the functional coordination of histone deaceylation and splicing. Mutations in histone residues targeted by Gcn5 show genetic interactions with the U2 snRNP and splicing defects that mirror GCN5 deletion. Furthermore, not only is Gcn5 associated throughout intron-containing genes, but deletion of multiple HDACs reveals peaks in acetylation in these regions, and this results in defects in spliceosome assembly. Finally, we present data that support a model whereby the Gcn5-dependent U2 snRNP recruitment facilitates HDAC recruitment, suggesting that splicing factors can, in fact, affect histone acetylation. These studies show that co-transcriptional spliceosome rearrangements are driven by dynamic changes in the acetylation state of histones and provide a model whereby spliceosome assembly is tightly coupled to histone modification