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 ADHI; ADR2, which encodes the glucose-repressed isozyme ADHII; 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. 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, and a number of cis-acting mutations identified that cause constitutive expression of ADR2, and seem to fall into two classes. 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. 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 cells. 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) X poly(T) tract 222 base pairs (bp) upstream of the gene.