The yeastPRS3 gene is required for cell integrity, cell cycle arrest upon nutrient deprivation, ion homeostasis and the proper organization of the actin cytoskeleton

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Abstract
PRS3 is one of a family of five genes encoding phosphoribosylpyrophosphate synthetase, an enzyme which catalyses the first step in a variety of biosynthetic pathways, including purine and pyrimidine biosynthesis. We report here that prs3Δ mutants have a number of phenotypes that suggest an unexpected role for PRS3 in linking nutrient availability to cell cycle progression, cell integrity and the actin cytoskeleton. Upon nutrient limitation, prs3Δ mutants fail to arrest in G1—cells remain budded and a significant fraction have a G2 DNA content. Furthermore, in such conditions, prs3Δ mutants have a disorganized actin cytoskeleton: actin accumulates in one or two intensely staining clumps per cell. Prs3Δ mutants also show defects in ion homeostasis and cell integrity. They fail to grow on medium containing 1·0 M NaCl, 5 mM caffeine or when incubated at 37°C. The caffeine and temperature sensitivity are rescued by supplementing the growth medium with 1·0 M sorbitol. These phenotypes resemble those of whi2Δ mutations and indeed, a prs3 allele was recovered in a colony‐sectoring screen for mutations that are co‐lethal with whi2Δ. However, further investigation showed that the prs3Δ whi2Δ double mutant was viable, with no obvious growth defect compared to either single mutant. In the same colony‐sectoring assay, an mpk1 allele was also recovered. Multicopy PRS3 rescued the caffeine sensitivity of this mpk1 allele. Copyright © 1999 John Wiley & Sons, Ltd.