IDENTIFICATION OF A PROTEIN THAT BINDS TO THE HO ENDONUCLEASE RECOGNITION SEQUENCE AT THE YEAST MATING-TYPE LOCUS

Mating type switching in Saccharomyces cerevisiae initiates when Ho en donuclease makes a site-specific double-stranded break at MAT, the yea st mating type locus. To identify other proteins involved in this proc ess, we examined whether extracts prepared from ho(-) mutants contain additional factors that bind near the recognition sequence for Ho. Usi ng an electrophoretic mobility shift assay, we isolated a chromatograp hic fraction that contains an activity, named YZbp, which binds to two sequences flanking the recognition sequence at MAT alpha and to one s equence overlapping it at MATa. MAT plasmids carrying mutations in the YZbp recognition sequence are cleaved by purified Ho at wild-type eff iciencies in an in vitro assay. These same plasmids, however, are not cleaved by Ho inside cells, demonstrating that YZbp acts as a positive activator of in vivo cleavage. YZbp is present in all cell types, eve n those not undergoing mating type switching, suggesting that it has a dditional cellular functions.