Heat shock promoters contain one or more binding sites for a specific heat shock factor (HSF). We report the cloning and sequence of the gene encoding yeast HSF, and demonstrate that HSF is required for growth at normal temperatures (15 degrees C-30 degrees C). The activity of a promoter containing a synthetic HSF binding site varies over a 200-fold range between 15 degrees C and 39 degrees C (heat shock). This change in activity is accompanied by multiple changes in the phosphorylation state of HSF, but all forms of HSF are able to bind DNA. We propose that the expression of heat shock genes in yeast is modulated by phosphorylation of DNA-bound HSF, and that this leads to a more efficient interaction of the factor with other components of the transcriptional machinery.