The Spliceosomal Protein SF3B5 is a Novel Component of Drosophila SAGA that Functions in Gene Expression Independent of Splicing.

The interaction between splicing factors and the transcriptional machinery provides an intriguing link between the coupled processes of transcription and splicing. Here, we show that the two components of the SF3B complex, SF3B3 and SF3B5, that form part of the U2 small nuclear ribonucleoprotein particle (snRNP) are also subunits of the Spt-Ada-Gcn5 acetyltransferase (SAGA) transcriptional coactivator complex in Drosophila melanogaster. Whereas SF3B3 had previously been identified as a human SAGA subunit, SF3B5 had not been identified as a component of SAGA in any species. We show that SF3B3 and SF3B5 bind to SAGA independent of RNA and interact with multiple SAGA subunits including Sgf29 and Spt7 in a yeast two-hybrid assay. Through analysis of sf3b5 mutant flies, we show that SF3B5 is necessary for proper development and cell viability but not for histone acetylation. Although SF3B5 does not appear to function in SAGA's histone-modifying activities, SF3B5 is still required for expression of a subset of SAGA-regulated genes independent of splicing. Thus, our data support an independent function of SF3B5 in SAGA's transcription coactivator activity that is separate from its role in splicing.