Genetics of alcohol dehydrogenase in Saccharomyces cerevisiae. II. Two loci controlling synthesis of the glucose-repressible ADH II.

Two unlinked loci controlling the glucose-repressible alcohol dehydrogenase (ADH II) in Saccharomyces cerevisiae were investigated. One locus (AD R2) was characterized by electrophoretically slow and fast alleles and by inactive adr2 mutant alleles. The ADH II pattern of heteroallelic slow X fast diploids indicates a tetrameric structure of the enzyme. AD R2 was considered as the structural gene, which codes for the ADH II subunits. Allelic adr2-f mutants could be classified by their response to the slow wild type allele (AD RS-S) in heterozygous diploids. In most cases, only the slow band appeared. In three adr2-f/ADR2-S crosses hybrid enzymes between inactive fast and active slow enzymes were formed. It was demonstrated, that allelic interactions at the protein level are not restricted to electrophoretical behaviour of hybrid enzymes. They also influence specific activities and substrate affinities. The other locus investigated, AD R1, was characterized by ADH II negative mutants (adr1) and by allelic mutants which generate only very low activity (ADR1-L). ADR1 does not influence the electrophoretic properties of slow and fast ADH II proteins. adr1 mutants have an intact structural gene, which is not expressed. The gene has probably a regulatory function with respect to ADH II synthesis.