Thermal response of yeast cells overexpressing hsp70 genes.

We have performed experiments to examine whether the overexpression of different members of the hsp70 family (SSA1, SSA2 and SSA4) in yeast Saccharomyces cerevisiae protects cells from thermal stress. Yeast cells were transformed with plasmids containing the SSA1 or SSA4 gene which was placed under the control of a galactose-inducible GAL1 promoter. In galactose, transformed yeast cells successfully overexpressed these hsp70 proteins at 23 degrees C, their normal growth temperature. When cell survival after heat shock at 50 or 56 degrees C was examined, our results showed that overexpression of SSA1 or SSA4 protein did not protect yeast cells against thermal stress, nor affect the cells' ability to develop thermotolerance. In contrast, thermal sensitivity was modified significantly by growing cells in galactose. Cellular survival of these cells after a 50 degrees C, 30-min heat treatment was 10(4)-fold higher than that of the same strains grown in glucose. In order to study the effects of overexpression of hsp70 on the thermal response of yeast cells independent of the carbon source, we eliminated the glucose-galactose-associated differences in thermal sensitivity by cloning the aforementioned hsp70 genes under the control of a glyceraldehyde phosphatase GAP promoter. S. cerevisiae transformed with these plasmids overexpressed the appropriate hsp70 gene in glucose as well as in galactose at 23 degrees C. Again, the overexpression of hsp70 neither protected cells from thermal stress, nor had a significant effect on the development of thermotolerance.