The characterisation of a multiple drug sensitive mutant of Saccharomyces cerevisiae

A Saccharomyces cerevisiae mutant, initially isolated as a novobiocin sensitive mutant, was shown to be sensitive to a wide range of drugs and chemicals. In order to identify the nature of the mutation (mds1) responsible for multiple drug-sensitivity, clones from a wild-type multicopy yeast genomic library conferring drug-resistance upon the mds1 strain were characterised and sequenced. From these, multicopy, extragenic suppressors and a likely candidate (SAC1) for the wild-type MDS1 gene were identified. MDS1 was shown to be allelic to SAC1 and thus, the drug sensitive phenotype was demonstrated to be a novel phenotype associated with mutations in SAC1.;SAC1 was originally isolated as a mutant suppressor of both actin (act1-1) and phosphatidylinositol/phosphatidylcholine transfer protein (PI-TP, sec14-1) mutations. Thus, a number of partially characterised mutant alleles were available for comparison with mds1.;Strains containing the mds1 allele could not suppress the effects of either act1-1 or sec14-1 mutations on the yeast cell, whereas other sac1 alleles could. Unlike other mutant alleles of sac1 the mds1 allele caused synthetic lethality with all tested actin mutations. Drug sensitivity was shown to be a feature of only some sac1 alleles. Some sac1 mutants were also shown to display inositol auxotrophy. It was considered that the bewildering set of phenotypes could be explained by modulation of phosphatidylinositol phosphate levels by Sac1p. Consequently phosphatidylinositol phosphate levels of a strain containing a null allele of the sac1 gene were tested and levels of phosphatidylinositol 4-phosphate were found to be elevated.;Based on these findings and the homology seen between a domain of polyphosphoinositide -5 phosphatases and Sac1p, a model has been developed which accounts for mediation by Sac1p of cytoskeletal and secretory functions concurrent with maintenance of plasma membrane integrity via modulation of phosphatidylinositol polyphosphates.