| Post-translational acetylation of chromatin is a key regulator of transcriptional activation, DNA repair, and genomic replication. The evolutionarily conserved SAGA (Spt-Ada-Gcn5 Acetyltransferase) and SLIK (SAGA- Like) histone acetyltransferase (HAT) complexes are required for transcriptional activation of a subset of yeast genes and contain multiple subunits including the histone fold-containing TBP-Associated Factors (TAFs): 6, 9, 10, and 12. These TAFs are also components of the TFIID complex and are consequently involved in most RNA polymerase II-mediated transcription in yeast. Here we identify a novel conserved domain of TAF12, outside of its histone fold, that is required for SAGA and SLIK-directed nucleosomal acetylation. We demonstrate that this domain is not required for chromatin association, but show that it plays an essential role in histone H3 acetylation at specific SAGA and SLIK-regulated promoters. These data suggest that the ReNu ( Required for Nucleosomal Acetylation) domain of TAF12 regulates Gcn5 acetylation of specific substrates by the SAGA super-family of HAT complexes. Additionally, we characterize a novel subunit, Ada6, shared between SAGA and SLIK. Ada6 mutations exhibit resistance to the toxic chimera GAL4-VP16 and reduced transcriptional capacity. Strikingly, Ada6-deleted SAGA and SLIK are not structurally compromised, but ada6 yeast are inviable on galactose, potassium acetate, and glycerol/ethanol carbon sources. Our data contrasts with reports demonstrating that only deletions of SAGA core structural components are inviable on galactose media. Our data suggest that the Ada6 module of the SAGA and SLIK complexes is required for transcriptional activation of galactose metabolism genes. |
