TOM1P, A YEAST HECT-DOMAIN PROTEIN WHICH MEDIATES TRANSCRIPTIONAL REGULATION THROUGH THE ADA SAGA COACTIVATOR COMPLEXES/

The hect-domain has been characterized as a conserved feature of a gro up of E3 ubiquitin Ligases. Here we show that the yeast hect-domain pr otein TOM1p regulates transcriptional activation through effects on th e ADA transcriptional coactivator proteins. Null mutations of tom1 res ult in similar defects in transcription from ADH2 and HIS3 promoters, and enhanced transcription from the GAL10 promoter as do null mutation s in ngg1/ada3. Strains with disruptions of both ngg1 and tom1 have th e same phenotype as strains with a disruption of only ng,ol implying t hat these genes are acting through the same pathway. In the absence of TOM1p the normal associations of the ADA proteins with SPT3p and the TATA-binding protein are reduced. The action of TOM1p is most likely m ediated through ubiquitination since mutation of Cys3235 to Ala, corre sponding residues of which are required for thioester bond formation w ith ubiquitin in other hect-domain proteins, results in similar change s in transcription as the null mutation. A direct role for TOM1p in re gulation of ADA-associated proteins is further supported by the findin g that SPT7p is ubiquitinated in a TOM1p-dependent fashion and that TO M1p coimmunoprecipitates with the ADA proteins. (C) 1998 Academic Pres s.