Regulation of basal and induced levels of the MEL1 transcript in Saccharomyces cerevisiae.

The MEL1 gene in Saccharomyces cerevisiae is required for the production of alpha-galactosidase and for the catabolism of melibiose. Production of alpha-galactosidase is induced by galactose or melibiose and repressed by glucose. Inducibility is controlled by the positive and negative regulatory proteins GAL4 and GAL80, respectively. We have cloned the MEL1 gene to study its transcriptional expression and regulation. Evidence is presented that the MEL1 gene encodes alpha-galactosidase and that mel0 is a naturally occurring allele which lacks the alpha-galactosidase-coding sequences. RNAs prepared from wild-type cells and from cells carrying either the noninducible gal4-2 or GAL80S-100 allele grown on three different carbon sources were examined by Northern hybridization analyses. In wild-type cells under noninducing conditions, such as growth on glycerol-lactic acid, the MEL1 transcript was detected at a basal level which was 1 to 2% of the fully induced level. The basal level of expression was diminished in cells carrying the gal4-2 mutant allele but not in cells carrying the GAL80S-100 allele. The basal and induced RNA levels are repressed by glucose. Size determinations of the MEL1 transcripts detected in glycerol-lactic acid- and galactose-grown cells provided no evidence for two distinct transcripts.