MUTATED YEAST HEAT-SHOCK TRANSCRIPTION FACTOR EXHIBITS ELEVATED BASALTRANSCRIPTIONAL ACTIVATION AND CONFERS METAL RESISTANCE

Cadmium-resistant Saccharomyces cerevisiae strain 301N exhibits high b asal as well as cadmium-induced expression of the CUP1 metallothionein gene. Since regulation of CUP1 is usually restricted to copper ions, our goal was to identify the factor responsible for the high metalloth ionein levels in strain 301N. The gene responsible for the observed ph enotype is a spontaneously mutated heat shock transcription factor gen e (HSF1). A double, semidominant HSF1 mutant with substitutions at cod ons 206 and 256 within the DNA-binding domain of the heat shock factor (HSF) confers two phenotypes. The first phenotype is elevated transcr iptional activity of the HSF mutant (HSF301), which results in constit utive thermotolerance. A second HSF301 phenotype is enhanced binding a ffinity for the heat shock element (HSE) within the CUP1 5'-sequences, resulting in high basal transcription of metallothionein. The CUP1 HS E is a minimal heat shock element containing only two perfectly spaced inverted repeats of the basic nGAAn block. Cells containing HSF301 ar e resistant to cadmium salts. The single R206S mutation is responsible for the high affinity binding to the CUP1 HSE. In addition, the R206S HSF substitution exhibits constitutive transcriptional activation fro m a consensus HSE (HSE2). The F256Y substitution in HSF attenuates the effects of R206S on the consensus HSE2, but not on the CUP1 HSE.