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We used mRNA differential display to assess yeast gene expression under cold or freeze shock stress conditions. We found
both up- and down-regulation of genes, although repression was more common. We identified and sequenced several
cold-induced genes exhibiting the largest differences. We confirmed, by Northern blotting, the specificity of the response for
TPII, which encodes trios e-phosphate isomerase; ERG10, the gene for acetoacetyl coenzyme A thiolase; and IMHI, which
encodes a protein implicated in protein transport. These genes also were induced under other stress conditions, suggesting that
this cold response is mediated by a general stress mechanism. We determined the physiological significance of the cold-induced
expression change of these genes in two baker's yeast strains with different sensitivities to freeze stress. The mRNA level of TPII
and ERG10 genes was higher in freeze-stressed than in control samples of the tolerant strain. In contrast, both genes were
repressed in frozen cells of the sensitive strain. Next, we examined the effects of ERG10 overexpression on cold and
freeze-thaw tolerance. Growth of wild-type cells at 10degreesC was not affected by high ERG10 expression. However,
YEpERG10 transformant cells exhibited increased freezing tolerance. Consistent with this, cells of an erg10 mutant strain
showed a clear phenotype of cold and freeze sensitivity. These results give support to the idea that a cause-and-effect
relationship between differentially expressed genes and cryoresistance exists in Saccharomyces cerevisiae and open up the
possibility of design strategies to improve the freeze tolerance of baker's yeast. |
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