GENETIC REDUNDANCY BETWEEN SPT23 AND MGA2 - REGULATORS OF TY-INDUCED MUTATIONS AND TY1 TRANSCRIPTION IN SACCHAROMYCES-CEREVISIAE

SPT23 was isolated as a dosage-dependent suppressor of Ty-induced muta tions in Saccharomyces cerevisiae. SPT23 shows considerable sequence h omology with MGA2, a gene identified as a dosage-dependent suppressor of a snf2-imposed block on STA1 transcription in S. cerevisiae var. di astaticus. Although single mutations in either of these genes have onl y modest effects on cell growth, spt23 mga2 double mutants are inviabl e. Unlike SPT23, multicopy expression of a truncated form of MGA2 supp resses a narrow subset of Ty-induced mutations. SPT23/MGA2 and the SNF /SWI genes affect transcription of certain target genes in similar way s, Spt23p appears to be a rate-limiting component required for functio nal HIS4 expression of his4-912 delta, a promoter insertion mutation i nduced by the Tyl-912 long terminal repeat, Furthermore, both Spt23p a nd Mga2p can activate transcription when fused to the Gal4p DNA-bindin g domain, as previously observed with Snf2p and Snf5p. A 50-amino-acid region in the N terminus of the predicted Spt23p protein is necessary and sufficient for the transactivation and necessary for suppression of Ty1-induced mutations and the essential function of Spt23p. Cell fr actionation and cytological experiments suggest that Spt23p is associa ted with the nucleus. Our results suggest that SPT23/MGA2 affects tran scription of a subset of genes in yeast, perhaps by changing chromatin accessibility.