A LOCUS-CONTROL REGION REGULATES YEAST RECOMBINATION

The yeast Saccharomyces can switch its mating type by a highly choreog raphed recombination event in which 'a' or 'alpha' sequences at the ma ting-type (MAT) locus are replaced by opposite mating-type sequences c opied from one of two donors, HML and HMR, located near the two ends o f the same chromosome III. MAT alpha cells 'know' to choose HML, while MAT alpha cells preferentially recombine with HMR. Donor preference i s regulated by a 250 bp recombination enhancer, that controls recombin ation of the entire left arm of chromosome III. Recent studies have sh own how this locus-control region is turned on and off.