Complex modes of heat shock factor activation.

Eucaryotic organisms respond to elevated environmental temperatures by rapidly activating the expression of heat shock genes. The transcriptional activation of heat shock genes is mediated by a conserved upstream regulatory sequence, the heat shock element (HSE). Using an HSE-binding assay, we show that a cellular factor present in a range of vertebrate species binds specifically to the HSE. This factor is presumably the transcriptional activator of heat shock genes, heat shock factor (HSF). In vertebrates, the binding of HSF to the HSE was induced when cells were subjected to heat shock at high temperatures, even in the absence of protein synthesis. Under mild heat shock conditions, HSF binding was induced to a lesser extent, but this induction required protein synthesis, suggesting that synthesis of HSF itself, or an activating factor, is necessary for response to heat shock at intermediate temperatures. The inducibility of HSF binding in higher eucaryotes is contrasted with constitutive HSF binding activity in fungi. It appears that despite conservation of the HSE in evolution, the means by which HSF is activated to bind DNA in higher and lower eucaryotes may have diverged.