Yeast GMP Kinase Mutants Constitutively Express AMP Biosynthesis Genes by Phenocopying a Hypoxanthine-Guanine Phosphoribosyltransferase Defect

Abstract

We have characterized a new locus, BRA3, leading to deregulation of the yeast purine synthesis genes (ADE genes). We show that bra3 mutations are alleles of the GUK1 gene, which encodes GMP kinase. The bra3 mutants have a low GMP kinase activity, excrete purines in the medium, and show vegetative growth defects and resistance to purine base analogs. The bra3 locus also corresponds to the previously described pur5 locus. Several lines of evidence indicate that the decrease in GMP kinase activity in the bra3 mutants results in GMP accumulation and feedback inhibition of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), encoded by the HPT1 gene. First, guk1 and hpt1 mutants share several phenotypes, such as adenine derepression, purine excretion, and 8-azaguanine resistance. Second, overexpression of HPT1 allows suppression of the deregulated phenotype of the guk1 mutants. Third, we show that purified yeast HGPRT is inhibited by GMP in vitro. Finally, incorporation of hypoxanthine into nucleotides is similarly diminished in hpt1 and guk1 mutants in vivo. We conclude that the decrease in GMP kinase activity in the guk1 mutants results in deregulation of the ADE gene expression by phenocopying a defect in HGPRT. The possible occurrence of a similar phenomenon in humans is discussed.

Changes in a Protein Profile Can Account for the Altered Phenotype of the Yeast Saccharomyces cerevisiae Mutant Lacking the Copper-Zinc Superoxide Dismutase

“…GUK1 is a guanylate kinase that converts guanosine monophosphate (GMP) to guanosine diphosphate (GDP). As shown earlier, the lack of the GUK1 gene led to GMP accumulation and, by a feedback mechanism, it inhibited hypoxanthine-guanine phosphoribosyltransferase activity (adenine derepression process; [ 59 ]). Therefore, the reduced expression of Guk1p may be another cause of the lower growth rate of the Δ sod1 mutant when compared to the WT strain.…”

Purine metabolism in plant pathogenic fungi

“…S. cerevisiae could take up extracellular purine bases and re-utilize nucleosides and nucleotides through the salvage pathway ( Kokina et al, 2019 ). In this process, xanthine-guanine phosphoribosyl transferase (Xpt1) is required for xanthine utilization and optimally utilization of guanine ( Guetsova et al, 1999 ), hypoxanthine-guanine phosphoribosyltransferase (Hpt1) is used to convert hypoxanthine and guanine to IMP and GMP, respectively ( Lecoq et al, 2000 ), and adenine phosphoribosyltransferase (Apt1) is recruited to convert adenine into AMP ( Alfonzo et al, 1995 ).…”