Yeast GMP kinase mutants constitutively express AMP biosynthesis genes by phenocopying a hypoxanthine-guanine phosphoribosyltransferase defect

We have characterized a new locus, BRA3 leading to deregulation of die yeas t purine synthesis genes (ADE genes). We show that bra3 mutations are allel es of the GUK1 gene, which encodes GMP kinase. The bra3 mutants have a low GMP kinase activity, excrete purines in die medium, and show vegetative gro wth defects and resistance to purine base analogs. The bra3 locus also corr esponds to the previously described pur5 locus. Several lines of evidence i ndicate that the decrease in CMP kinase activity in the bra3 mutants result s in GMP accumulation and feedback inhibition of hypoxanthine-guanine phosp horibosyltransferase (HGPRT), encoded by the HPT1 gene. First, guk1 and hpt 1 mutants share several phenotypes, such as adenine derepression, purine ex cretion, and 8-azaguanine resistance. Second, overexpression of HPT1 allows suppression of the: deregulated phenotype of the guk1 mutants. Third, we s how that purified yeast HGPRT is inhibited by GMP in vitro. Finally, incorp oration of hypoxanthine into nucleotides is similarly diminished in hpt1 an d guk1 mutants in vivo. We conclude that the decrease in GMP kinase activit y in the guk1 mutants results in deregulation of the ADE gene expression by phenocopying a defect in HGPRT. The possible occurrence of a similar pheno menon in humans is discussed.