Required, directly or indirectly, for TATA-binding protein function at particular promoters. May promote a functional interaction between SPT3 and TATA-binding protein. Functions as a component of the transcription regulatory histone acetylation (HAT) complex SAGA. SAGA is involved in RNA polymerase II-dependent transcriptional regulation of approximately 10% of yeast genes. At the promoters, SAGA is required for recruitment of the basal transcription machinery. It influences RNA polymerase II transcriptional activity through different activities such as TBP interaction (SPT3, SPT8 and SPT20) and promoter selectivity, interaction with transcription activators (GCN5, ADA2, ADA3 and TRA1), and chromatin modification through histone acetylation (GCN5) and deubiquitination (UBP8). SAGA acetylates nucleosomal histone H3 to some extent (to form H3K9ac, H3K14ac, H3K18ac and H3K23ac). SAGA interacts with DNA via upstream activating sequences (UASs). Component of the 1.8 MDa SAGA complex, which consists of at least of TRA1, CHD1, SPT7, TAF5, ADA3, SGF73, SPT20/ADA5, SPT8, TAF12, TAF6, HFI1/ADA1, UBP8, GCN5, ADA2, SPT3, SGF29, TAF10, TAF9, SGF11 and SUS1. TAF5, TAF6, TAF9, TAF19, TAF12 and ADA1 seem to be present in 2 copies. SAGA is built of 5 distinct domains with specialized functions. Domain I (containing TRA1) probably represents the activator interaction surface. Domain II (containing TAF5 and TAF6, and probably TAF9 and TAF10), domain III (containing GCN5, TAF10, SPT7, TAF5 and ADA1, and probably ADA2, ADA3 and TAF12), and domain IV (containing HFI1/ADA1 and TAF6, and probably TAF9) are believed to play primarily an architectural role. Domain III also harbors the HAT activity. Domain V (containing SPT3 and SPT20, and probably SPT8) represents the TBP-interacting module, which may be associated transiently with SAGA. Present with 8430 molecules/cell in log phase SD medium. Belongs to the WD repeat SPT8 family.