Isolation and characterization of STI1, a stress-inducible gene from Saccharomyces cerevisiae.

We have isolated a gene from the yeast Saccharomyces cerevisiae that encodes a 2.0-kilobase heat-inducible mRNA. This gene, which we have designated STI1, for stress inducible, was also induced by the amino acid analog canavanine and showed a slight increase in expression as cells moved into stationary phase. The STI1 gene encodes a 66-kilodalton protein, as determined from the sequence of the longest open reading frame. The putative STI1 protein, as identified by two-dimensional gel electrophoresis, migrated in the region of 73 to 75 kilodaltons as a series of four isoforms with different isoelectric points. STI1 is not homologous to the other conserved HSP70 family members in yeasts, despite similarities in size and regulation. Cells carrying a disruption mutation of the STI1 gene grew normally at 30 degrees C but showed impaired growth at higher and lower temperatures. Overexpression of the STI1 gene resulted in substantial trans-activation of SSA4 promoter-reporter gene fusions, indicating that STI1 may play a role in mediating the heat shock response of some HSP70 genes.     

Positive and negative regulation of basal expression of a yeast HSP70 gene.

The SSA1 gene, one of the heat-inducible HSP70 genes in the yeast Saccharomyces cerevisiae, also displays a basal level of expression during logarithmic growth. Multiple sites related to the heat shock element (HSE) consensus sequence are present in the SSA1 promoter region (Slater and Craig, Mol. Cell. Biol. 7:1906-1916, 1987). One of the HSEs, HSE2, is important in the basal expression of SSA1 as well as in heat-inducible expression. A promoter containing a mutant HSE2 showed a fivefold-lower level of basal expression and altered kinetics of expression after heat shock. A series of deletion and point mutations led to identification of an upstream repression sequence (URS) which overlapped HSE2. A promoter containing a mutation in the URS showed an increased level of basal expression. A URS-binding activity was detected in yeast whole-cell extracts by a gel electrophoresis DNA-binding assay. The results reported in this paper indicate that basal expression of the SSA1 promoter is determined by both positive and negative elements and imply that the positively acting yeast heat shock factor HSF is responsible, at least in part, for the basal level of expression of SSA1.