Activation of heat shock transcription factor 1 to a DNA binding form during the G(1) phase of the cell cycle

The heat shock transcription factor (HSF) genes encode proteins that bind t o the heat shock elements (HSE) of stress-inducible genes. We have observed the induction of HSF1, the ubiquitous member of the HSF family from a late nt cytoplasmic state to a form competent to bind HSE during early G(1) in H eLa cells in the absence of stress. The induction of DNA-binding HSF1 coinc ided with a burst in cellular protein synthesis in early G(1) and inhibitio n of this translational peak prevented the formation of DNA binding-activat ed HSF1. A potential role for HSF1 in cell cycle regulation was suggested b y the finding that cell lines stably overexpressing HSF1 showed an increase d proportion of G(1) cells relative to other cell cycle phases. However, in contrast to the effects of heat shock, entry into G(1) did not lead to HSF 1 hyperphosphorylation or increased activity of a heat shock promoter-repor ter gene and did not cause the induction of heat shock protein 70 expressio n. Thus HSF1, previously implicated in the heat shock response is activated to a DNA binding from in G(1) under non-stress conditions and may play a r ole in G(1) regulation that does not involve the transcription of heat shoc k genes.