MUTATIONS SENSITIZING YEAST-CELLS TO THE START INHIBITOR NALIDIXIC-ACID

The regulatory step Start in the cell cycle of the budding yeast Sacch aromyces cerevisiae is inhibited by nalidixic acid (Nal). To study thi s inhibition, mutations were identified that alter the sensitivity of yeast cells to Nal. Nal-sensitive mutations were sought because the in hibitory effects of Nal on wild-type cells are only transient, and wil d-type cells naturally become refractory to Nal. Three complementation groups of Nal-sensitive mutations were found. Mutations in the first complementation group were shown to reside in the ARO7 gene, encoding chorismate mutase; tyrosine and phenylalanine synthesis was inhibited by Nal in these aro7 mutants, whereas wild-type chorismate mutase was unaffected. These aro7 alleles demonstrate 'recruitment', by mutation, of an innately indifferent protein to an inhibitor-sensitive form. Th e Nal-sensitive aro7 mutant cells were used to show that the resumptio n of Nal-inhibited nuclear activity and cell proliferation takes place while cytoplasmic Nal persists at concentrations inhibitory for the m utant chorismate mutase. Mutations in the second complementation group , nss2 (Nal-supersensitive), increased intracellular Nal concentration s, and may simply alter the permeability of cells to Nal. The third co mplementation group was found to be the ERG6 gene, previously suggeste d to encode the ergosterol biosynthetic enzyme sterol methyltransferas e. Mutation or deletion of the ERG6 gene had little effect on the inhi bition of Start by Nal, but prevented recovery from this inhibition. M utation of ERG3, encoding another ergosterol biosynthetic enzyme, also caused Nal sensitivity, suggesting that plasma membrane sterol compos ition, and plasma membrane function, mediates recovery from Nal-mediat ed inhibition of Start.