Functions as a component of the DNA-binding general transcription factor complex TFIID and the transcription regulatory histone acetylation (HAT) complexes SAGA, SALSA and SLIK. Binding of TFIID to a promoter (with or without TATA element) is the initial step in preinitiation complex (PIC) formation. TFIID plays a key role in the regulation of gene expression by RNA polymerase II through different activities such as transcription activator interaction, core promoter recognition and selectivity, TFIIA and TFIIB interaction, chromatin modification (histone acetylation by TAF1), facilitation of DNA opening and initiation of transcription. 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). SALSA, an altered form of SAGA, may be involved in positive transcriptional regulation. SLIK is proposed to have partly overlapping functions with SAGA. It preferentially acetylates methylated histone H3, at least after activation at the GAL1-10 locus. In TFIID, TAF12 heterodimerizes with TAF4, forming ultimately an octamer consisting of a TAF6/TAF9 heterotetramer core flanked by TAF4/TAF12 dimers on either side, similar to the histone H2A/H2B/H3/H4 octamer. The 1.2 MDa TFIID complex is composed of TATA binding protein (TBP) and the 14 TBP-associated factors. One copy of each TAF1, TAF2, TAF3, TAF7, TAF8, TAF11, TAF13, two copies of each TAF4, TAF5, TAF6, TAF9, TAF10, TAF12, and three copies of TAF14. Component of the 1.8 MDa SAGA complex, which consists of at least 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. Component of the SALSA complex, which consists of at least TRA1, SPT7 (C-terminal truncated form), TAF5, ADA3, SPT20, TAF12, TAF6, HFI1, GCN5, ADA2 and SPT3. Component of the SLIK complex, which consists of at least TRA1, CHD1, SPT7, TAF5, ADA3, SPT20, RTG2, TAF12, TAF6, HFI1, UBP8, GCN5, ADA2, SPT3, SGF29, TAF10 and TAF9. Present with 930 (+/-45) molecules/cell in log phase SD medium. Belongs to the TAF12 family.