Constitutive Mutations of the Saccharomyces cereviSiae MAGActivator Genes MAL 23 , MAL 43 , MAL 63 , and ma 164 Andrew

We report the sequence of several MALactivator genes, including inducible, constitutive, and noninducible alleles of MAL23, MAL43, MAL63, and ma164. Constitutive alleles of MAL23 and MAL43 vary considerably from inducible alleles in their C-terminal domain, with many of the alterations clustered and common to both alleles. The 27 alterations from residues 238-461 of Ma143-C protein are sufficient for constitutivity, but the minimal number of alterations needed for the constitutive phenotype could not be determined. The sequence of ma164, a nonfunctional homologue of MAL63, revealed that Ma164p is 85% identical to Ma163p. Two mutations that activate ma164 and cause constitutivity are nonsense mutations resulting in truncated proteins of 306 and 282 residues. We conclude that the C-terminal region of the MALactivator, from residues 283-470, contains a maltose-responsive negative regulatory domain, and that extensive mutation or deletion of the entire region causes loss of the negative regulatory function. Additionally, certain sequence elements in the region appear to be necessary for efficient induction of the full-length Ma163 activator protein. These studies highlight the role of ectopic recombination as an important mechanism of mutagenesis of the telomere-associated family of MAL loci. M ALTOSE fermentation in Saccharomyces requires the presence of one of five unlinked MAL loci: M A L I , MAL2, MAL3, MAL4 and MAL6 (reviewed in NEEDLEMAN 1991). Each locus encodes three gene products essential for maltose fermentation. Genes 1 and 2 encode maltose permease and maltase, respectively; gene 3 encodes the MAL transcriptional activator protein (HONG and MARMuR 1986; KIM and MICHELS 1988; CHENG and MICHELS 1989). At MAL6, the three genes are referred to as MAL61, MAL62, and MAL63 (see Figure 1). Transcription of the structural genes is induced by maltose and repressed by glucose (NEEDLEMAN et al. 1984; CHARRON et al. 1986; LEVINE et al. 1992; HU et al. 1995). The MALactivator mediates both regulatory processes but is only one of several factors controlling glucose repression of maltose fermentation