DNA sequences of two yeast promoter-up mutants

The yeast Saccharomyces cerevisiae has three genetic loci encoding different alcohol dehydrogenase (ADH) isozymes: ADC1, which encodes the classical fermentative isozyme ADHI¹; ADR2, which encodes the glucose-repressed isozyme ADHII²; and ADM, which encodes an ADH isozyme found associated with mitochondria. When yeast are grown on glucose, the ADC1 gene is expressed, and the ADR2 gene repressed^1,3,4. Conversely, growth on a non-fermentable carbon source such as ethanol or glycerol results in derepression of ADR2, and repression of ADC1. The ADC1 and ADR2 genes have been cloned and sequenced^5-8, and a number of cis-acting mutations identified that cause constitutive expression of ADR2 ^9,10, and seem to fall into two classes¹¹. The most abundant class consists of mutants that cannot be fully derepressed, and do not revert to wild type at a detectable level: these are caused by the insertion of a transposable element into the 5′-flanking region of the gene ^6,12. The second class of mutants do revert to a glucose-repressed phenotype at a detectable frequency, and when grown on non-fermentable carbon sources derepress ADR2 to levels up to five times those found in wild-type cells¹⁰. We report here the sequencing of the 5′-flanking regions of two such promoter-up, constitutive ADR2 mutants, in both of which the mutant phenotype is associated with an increase in length of a poly(A)·poly(T) tract 222 base pairs (bp) upstream of the gene.