Functional organization of the yeast SAGA complex: Distinct components involved in structural integrity, nucleosome acetylation, and TATA-binding protein interaction

SAGA, a recently described protein complex in Saccharomyces cerevisiae, is important for transcription in vivo and possesses histone acetylation funct ion. Here we report both biochemical and genetic analyses of members of thr ee classes of transcription regulatory factors contained within the SAGA co mplex. We demonstrate a correlation between the phenotypic severity of SAGA mutants and SAGA structural integrity. Specifically, null mutations in the Gcn5/Ada2/Ada3 or Spt3/Spt8 classes cause moderate phenotypes and subtle s tructural alterations, while mutations in a third subgroup, Spt7/Spt20, as well as Ada1, disrupt the complex and cause severe phenotypes. Interestingl y, double mutants (gcn5 Delta spt3 Delta and gcn5 Delta spt8 Delta) causing loss of a member of each of the moderate classes have severe phenotypes, s imilar to spt7 Delta, spt20 Delta, or ada1 Delta mutants. In addition, we h ave investigated biochemical functions suggested by the moderate phenotypic classes and find that first, normal nucleosomal acetylation by SAGA requir es a specific domain of Gcn5, termed the bromo-domain. Deletion of this dom ain also causes specific transcriptional defects at the HIS3 promoter in vi vo. Second, SAGA interacts with TBP, the TATA-binding protein, and this int eraction requires Spt8 in vitro. Overall, our data demonstrate that SAGA ha rbors multiple, distinct transcription-related functions, including direct TBP interaction and nucleosomal histone acetylation. Loss of either of thes e causes slight impairment in vivo, but loss of both is highly detrimental to growth and transcription.