| Genetic studies in yeast have identified many factors important for regulation of transcription initation by RNA polymerase II. One group of factors, encoded by SPT genes, are hypothesized to help the TATA binding protein (TBP) function at particular promoters. To identify additional related functions, we selected for new spt mutations and identified a novel gene, SPT20. Mutations in SPT20 cause mutant phenotypes and transcriptional defects similar to those caused by mutations in SPT3, SPT7, SPT8, and SPT15 (which encodes TBP), suggesting that Spt20 is functionally related to these factors. Interestingly, SPT20 is identical to ADA5. Ada proteins, Ada2, Ada3, and Gcn5, have been proposed to encode adaptors that bridge general transcription factors and activator proteins. Gcn5 was recently identified as a histone acetyltransferase, suggesting a link between Spt and Ada proteins, and histone acetylation which has been correlated with increased transcription. In support of this hypothesis, we present evidence for physical association of Spt20 with three other Spt proteins, suggesting that they exist in a complex in vivo. In collaboration, we observe that this complex, named SAGA, contains at least two additional proteins, Gcn5 and Ada2, and possesses histone acetyltransferase activity.;To identify functions that genetically interact with SAGA, we screened for mutations that cause lethality with an spt20$Delta$ mutation. Our screen identified mutations that affect two previously characterized transcription complexes, Snf/Swi, which functions in nucleosome remodeling, and Srb/Mediator, required for activated transcription by RNA polymerase II. Genetic analysis suggests that SAGA carries out multiple functions that are overlapping with Snf/Swi and Srb/mediator, and that most of these functions are dependent on Spt20 while only a subset are dependent on Gcn5.;Thus, in vivo it appears that cells require the functions of SAGA, Snf/Swi, and Srb/mediator for optimal transcription. Because proper regulation of transcription is critical for cell viability, organisms may possess these partially redundant functions to ensure that this process occurs correctly. |
