REMOVAL OF A MIG1P BINDING-SITE CONVERTS A MAL63 CONSTITUTIVE MUTANT DERIVED BY INTERCHROMOSOMAL GENE CONVERSION TO GLUCOSE INSENSITIVITY

Maltose fermenting strains of Saccharomyces cerevisiae have one or mor e complex loci called MAL. Each locus comprises at least three genes: MALx1 encodes maltose permease, MALx2 encodes maltase, and MALx3 encod es an activator of MALx1 and MALx2 (x denotes one of five MAL loci, wi th x = 1, 2, 3, 4, or 6). The MAL43(c) allele is constitutive and rela tively insensitive to glucose repression. To understand better this un ique phenotype of MAL43(c), we have isolated several MAL63(c) constitu tive mutants from a MAL6 strain. All constitutive mutants remain gluco se repressible, and all have multiple amino acid substitutions in the C-terminal region, now making this region of Mal63(c)p similar to that of Mal43(c)p. These changes have been generated by gene conversion, w hich transfers DNA from the telomeres of chromosome II and chromosome III or XVI to chromosome VIII (MAL6). The removal of a Mig1p binding s ite from the MAL63(c) promoter leads to a loss of glucose repression, imitating the phenotype of MAL43(c). Conversely, addition of a Mig1p b inding site to the promoter of MAL43(c) converts it to glucose sensiti vity. Mig1p modulation of Mal63p and Mal43p expression therefore plays a substantial role in glucose repression of the MAL genes.