Induction of heat shock genes is mediated by heat shock factor (HSF). Our recent genomic footprinting experiments demonstrate that HSF binds constitutively to perfect and imperfect heat shock elements (HSEs) in the HSP26 gene in yeast. Site-directed mutagenesis of the single perfect HSE, previously reported to not be involved in regulating gene expression, significantly reduces the rate of response of the gene to heat shock. However, the same mutation only slightly reduced the rate of accumulation of HSP26 mRNA during heat shock. Genomic footprinting experiments indicate that this lag in response to heat shock is due to the failure of HSF to bind efficiently to the mutated HSE. The rate of response to heat shock of synthetic promoters containing one, two, three, or seven perfect HSEs was similar to that observed for the wild-type HSP26 gene. These results suggest that the rate of response to heat shock is correlated with HSF occupancy of HSEs, rather than the number of HSEs in a promoter. As with the wild-type and mutant HSP26 genes, the rate of accumulation of mRNA from synthetic promoters increased only moderately with an increase in the number of HSEs. These results suggest that as few as two HSE-HSF complexes are sufficient to saturate HSF's target in the basal transcription apparatus.