Functions as histone deubiquitinating component of the transcription regulatory histone acetylation (HAT) complexes SAGA and SLIK. 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). 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. Together with SGF11, is required for histone H2B deubiquitination. Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). 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. UBP8 interacts with SGF11. Component of the SLIK complex, which consists of at least TRA1, CHD1, SPT7, TAF5, ADA3, SPT20, RTG2, TAF12, TAF6, HFI1, UBP8, GCN5, ADA2, SPT3, TAF10 and TAF9. The UBP-type zinc finger domain is required for the interaction with the SAGA complex. Present with 1030 molecules/cell in log phase SD medium. Belongs to the peptidase C19 family. UBP8 subfamily.