The Saccharomyces cerevisiae Mutation elm4-1 Facilitates Pseudohyphal Differentiation and Interacts with a Deficiency in Phosphoribosylpyrophosphate Synthase Activity To Cause Constitutive

Saccharomyces cerevisiae mutant E124 was selected in a visual screen based on elongated cell shape. Genetic analysis showed that E124 contains two separate mutations, ppsl-1 and elm4-1, each causing a distinct phenotype inherited as a single-gene trait. In rich medium, ppsl-i by itself causes increased doubling time but does not affect cell shape, whereas eln4-1 results in a moderate cell elongation phenotype but does not affect growth rate. Reconstructed ebn4-1 ppsi-i double mutants display a synthetic phenotype in rich medium including extreme cell elongation and delayed cell separation, both characteristics of pseudohyphal differen-tiation. The elm4-1 mutation was shown to act as a dominant factor that potentiates pseudohyphal differentiation in response to general nitrogen starvation in a genetic background in which pseudohyphal growth normally does not occur. Thus, elm4-1 allows recognition of, or response to, a pseudohyphal differentiation signal that results from nitrogen limitation. PPS1 was isolated and shown to be a previously undescribed gene coding for a protein similar in amino acid sequence to phosphoribosylpyrophosphate synthase, a rate-limiting enzyme in the biosynthesis of nucleotides, histidine, and tryptophan. Thus, the ppsl-1 mutation may generate a nitrogen limitation signal, which when coupled with elm4-1 results in pseudohyphal growth even in rich medium. The yeast Saccharomyces cerevisiae is a dimorphic organis