SPT3 encodes a subunit of SAGA, a multi-subunit complex that either activates or inhibits transcription of some RNA polymerase II-dependent genes (4). In its role as an activator, Spt3p interacts with the TATA-binding protein (TBP), encoded by SPT15 (3), and has been shown to be required for recruitment of TBP to the SAGA-dependent promoters GAL1 (5, 6), VTC3, BDF2, PHO84, and ADH1 (7). Studies of activation at the GAL1 promoter indicate that the activator protein Gal4p recruits SAGA to the upstream activation sequence (UAS), and that SAGA subsequently recruits TBP via Spt3p and Spt20p (6). During SAGA-mediated transcriptional inhibition, which has been demonstrated at the HIS3 and TRP3 promoters, Spt3p and Spt8p prevent TBP binding to the TATA box (2).

SPT3 was originally identified in a genetic screen for mutations that suppress Ty or solo Delta element insertions in the HIS4 or LYS2 promoters (8, 1). Null mutations in SPT3 confer defects in mating (9), sporulation (10), diploid pseudohyphal growth, and haploid invasive growth (11). SPT3 homologs have been identified in humans (12, 13) and other fungi (14, 11). Candida albicans SPT3, which complements an S. cerevisiae spt3 null mutation, is required for normal filamentation and for virulence in C. albicans . Interestingly, null mutations in C. albicans SPT3 cause hyperfilamentation, the opposite effect of that observed in S. cerevisiae spt3 null mutants (11).

Characterized subunits of the SAGA complex include: Hfi1p, Ada2p, Ngg1p, Spt20p, Gcn5p, Spt3p, Spt7p, Spt8p, Tra1p, Taf5p, Taf6p, Taf9p, Taf10p, Taf12p, Ubp8p, and Sgf11p (15, 16; reviewed in 17). Cells also contain an altered form of SAGA, referred to as SLIK (SAGA-like) or SALSA (SAGA altered, Spt8 absent), which is also involved in transcription activation (18, 17). This complex contains Spt3p, but it lacks Spt8p and has a truncated form of Spt7p.

Last updated: 2006-07-03