Strains of that exhibit varied capacities for accumulation of trehalose were tested for intrinsic thermotolerance. Yeast that accumulated trehalose rapidly in early respiratory phase showed equally rapid attainment of thermotolerance, whereas a strain unable to accumulate trehalose at this stage of culture showed markedly delayed appearance of thermotolerance. These results were obtained using closely related but non-isogenic diploids and so it is possible that variable factors other than trehalose were responsible for the observed thermotolerance effects. Therefore, a pair of isogenic diploid strains was generated to facilitate further testing of whether trehalose functions in intrinsic stress tolerance. Both isogenic strains inherited a partially reverted phenotype, designated CPR, from the trehalose-deficient progenitor that had been used in construction of the non-isogenic strains. The CPR phenotype permitted growth on glucose but not accumulation of trehalose, indicating that not all -related deficiencies were suppressed in the CPR strains. However, one of the isogenic CPR pair was and failed to accumulate trehalose, whilst the other was and was able to accumulate this sugar. The trehalose-proficient strain showed intrinsic stress tolerance whereas the trehalose-deficient strain was sensitive to heat stress during early respiratory growth. These results suggest that one or more functions of , not operating in the (CPR) strains, are important for intrinsic thermotolerance of yeast in early respiratory phase. When considering these results with those of others whose work has indicated a role for trehalose in protection of proteins and membranes, it is reasonable to hypothesize that the trehalose deficiency associated with (CPR) strains could be a key factor in their intrinsic thermosensitivity. However, if this is the case the importance of trehalose, relative to other stress tolerance factors, appears to vary with growth phase and culture status.


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