EVIDENCE FOR THE INVOLVEMENT OF VACUOLAR ACTIVITY IN METAL(LOID) TOLERANCE - VACUOLAR-LACKING AND VACUOLAR-DEFECTIVE MUTANTS OF SACCHAROMYCES-CEREVISIAE DISPLAY HIGHER SENSITIVITY TO CHROMATE, TELLURITE AND SELENITE

The responses of Saccharomyces cerevisiae towards the oxyanions tellur ite, selenite and chromate were investigated in order to establish the involvement of the yeast vacuole in their detoxification. Three mutan ts of S. cerevisiae with defective vacuolar morphology and function we re used; mutant JSR180 Delta 1 is devoid of any vacuolar-like structur e while ScVatB and ScVatC are deficient in specific protein subunits o f the vacuolar (V)-H+-ATPase. All the mutant strains showed increased sensitivity to tellurite and chromate compared to their parental strai ns. Such sensitivity of the mutants was associated with increased accu mulation of tellurium and chromium. These results indicate that accumu lation of both tellurium and chromium occurred mainly in the cytosolic compartment of the cell, with detoxification influenced by the presen ce of a functionally-active vacuole which may play a role in compartme ntation as well as regulation of the cytostolic compartment for optima l expression of a detoxification mechanism, e. g. reduction. In contra st, the vacuolar-lacking mutant, JSR180 Delta 1, and the defective V-H (+)ATPase mutant ScVatB displayed lower selenium accumulation than the ir parental strains. Additionally, the mutant strain ScVatB displayed a higher tolerance to selenite than the parental strain. This result s uggests that accumulation of selenium occurs mainly in the vacuolar co mpartment of the cell with tolerance depending on the ability of the c ytosolic component to reduce selenite to elemental selenium, which mig ht, in turn, be related to activity of the V-H+-ATPase. These results are discussed in relation to vacuolar compartmentation and the signifi cance of the vacuolar H+-ATPase in cytosolic homeostasis of H+ both of which may affect the accumulation, reduction, and tolerance to the te sted metal(loids).