Complex interactions among members of an essential subfamily of hsp70 genes in Saccharomyces cerevisiae.

Saccharomyces cerevisiae contains a large family of genes related to hsp70, the major heat shock-inducible gene of Drosophila melanogaster. One subfamily, identified by sequence homology, contains four genes, SSA1, SSA2, SSA3, and SSA4 (formerly YG100, YG102, YG106, and YG107, respectively). Previous studies showed that strains containing mutations in SSA1 and SSA2 are temperature sensitive for growth. SSA4, which is normally heat inducible and not expressed during vegetative growth, is expressed at high levels in ssa1 ssa2 strains at 23 degrees C. We constructed mutations in SSA3 and SSA4 and analyzed strains carrying mutations in the four genes. Strains carrying mutations in SSA3 SSA4 or SSA3 and SSA4 were indistinguishable from the wild type. However, ssa1 ssa2 ssa4 strains were inviable. SSA3, like SSA4, is a heat-inducible gene that is not normally expressed at 23 degrees C. Nevertheless, an intact copy of SSA3 regulated by the constitutive SSA2 promoter was capable of rescuing a ssa1 ssa2 ssa4 strain. This indicates that SSA3 encodes a functional protein and that the SSA1, SSA2, SSA3, and SSA4 gene products are functionally similar.