THE VACUOLAR COMPARTMENT IS REQUIRED FOR SULFUR AMINO-ACID HOMEOSTASIS IN SACCHAROMYCES-CEREVISIAE

In order to isolate new mutations impairing transcriptional regulation of sulfur metabolism in Saccharomyces cerevisiae, we used a potent ge netic screen based on a gene fusion expressing XylE (from Pseudomonas putida) under the control of the promoter region of MET25. This select ion yielded strains mutated in Various different genes. We describe in this paper the properties of one of them, MET27. Mutation or disrupti on of MET27 leads to a methionine requirement and affects S-adenosylme thionine (AdoMet)-mediated transcriptional control of genes involved i n sulfur metabolism. The cloning and sequencing of MET27 showed that i t is identical to VPS33. Disruptions or mutations of gene VPS33 are we ll known to impair the biogenesis and inheritance of the vacuolar comp artment. However, the methionine requirement of vps33 mutants has not been reported previously. We show here, moreover, that other vps mutan ts of class C (no apparent vacuoles) also require methionine for growt h. Northern blotting experiments revealed that the met27-1 mutation de layed derepression of the transcription of genes involved in sulfur me tabolism. By contrast, this delay was not observed in a met27 disrupte d strain. Physiological and morphological analyses of met27-1 and met2 7 disrupted strains showed that these results could be explained by al terations in the ability of the vacuole to transport or store AdoMet, the physiological effector of the transcriptional regulation of sulfur metabolism.