Cloning and characterization of the CYC8 gene mediating glucose repression in yeast

Abstract

Mutations m the CYC8( = SSN6) gene of Saccharomyces cerevisiae alleviate glucose repression of many glucose-repressible genes. The gene was isolated by screening for complementation of a cyc8 effect on colony morphology. Subclones containing a 5.3-kb SalI- XbaI fragment provided complete complementation. The gene was further localized to 3.5 kb by mapping of the CYC8 mRNA and insertional mutagenesis. Insertion and deletion mutations are viable and produce the same array of phenotypes as point mutations. CYC8 disruptions also had effects on the mating ability and morphology of MATα cells similar to that of tup1 mutations. The nucleotide sequence of a 4866-bp fragment, including CYC8, was determined. One long open reading frame of 966 amino acids predicts a protein of molecular weight 107215. The predicted protein is extremely glutamine-rich, with blocks of 16 and 31 glutamines in tandem at the N and C regions, respectively. The CYC8 gene product lacks consensus sequences for DNA-binding domains, suggesting that its function may be different from classical represser proteins.