Functions as component of the transcription regulatory histone acetylation (HAT) complex SAGA. SAGA is involved in RNA polymerase II-dependent transcriptional regulation. 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). Spt3 is required for recruitment of TATA-binding protein (TBP) to SAGA-dependent promoters. During SAGA-mediated transcriptional inhibition, spt3 and spt8 prevent binding of TBP to the TATA box (By similarity). SPT factors 3, 7 and 8 are required for the initiation of Ty transcription from the delta promoter. Spt3 regulates Ty1 as well as the mating factor genes. Component of the SAGA complex. Belongs to the SPT3 family.