The formation of ADHII in Saccharomyces cerevisiae is regulated by carbon catabolite repression. There are two genes involved in the formation of ADHII: ADR2, the structural gene as identified by electrophoretic variants and ADR1, possibly a regulatory gene. A new genetic element involved in the regulation of ADHII was identified by three allelic mutants insensitive to strong glucose repression. They were called ADR3c (wild type designation ADR3) and found to be tightly linked to the structural gene, ADR2. The alcohol dehydrogenase found in ADR3c mutants could not be distinguished electrophoretically from the ADHII of the glucose-sensitive wild type, ADR3. Dominance relations between ADR3c and ADR3 were established in diploids heterozygous for ADR3 and the two alleles of ADR2 (ADR2-S: slow ADHII, ADR2-F: fast ADHII). During growth on 10% glucose, an ADR3c adr2-F/ADR3 ADR2-Sheterozygous diploid formed only the fast ADHII variant wheras an ADR3c ADR2-S/ADR3 ADR2-F heterozygote produced only the slow form. This was taken as evidence of the cis-dominance of all ADR3c alleles. The cis-effect of ADR3c was also demonstrated in glucose-derepressed diploids. The ADR3c mutations do not only cause glucose-insensitive ADHII frmation, but also reduce the activity of the adjacent structural gene during derepression. Thus ADR3c alleles were considered to be controlling site mutations. No pleiotropic effects were observed on the formation of enzymes related to the function of ADHII. An adr1 ADR2 ADR3 single mutant did not form ADHII. In contrast to this, an adr1 ADR2 ADR3c double mutant formed ADHII at a similar level as double mutant formed ADHII at a similar level as an ADR1 ADR2 ADR3c mutant. This showed that ADR3c was epistatic over adr1 (previously suggested as a positive regulatory gene). From this it was concluded that ADR1 is the fact a positive regulatory gene the function of which is required for the expression of the structural gene for ADHII, ADR2. ADR3 is the controlling site for the structural gene ADR2. Mutations at this site, ADR3c, alleviate the requirement for the ADR2 gene product. Adr3c is discussed as a promotor or operator site.

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Ciriacy M

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