DNA supercoiling and temperature shift affect the promoter activity of the Escherichia coli rpoH gene encoding the heat-shock sigma subunit of RNA polymerase.

The effects of DNA supercoiling and temperature shift on the activity of two major promoters of the Escherichia coli rpoH gene, which codes for the heat-shock sigma subunit of RNA polymerase, were examined using an in vitro transcription system. Upstream promoter P1 is not affected by DNA supercoiling nor temperature upshift but downstream promoter P2 is enhanced by both of these factors. Both factors facilitate the formation of an open complex on rpoH P2, and these two effects are not additive. Hence these two factors act at the same step of transcription, probably the local unwinding of the DNA double helix at the promoter region. The different sensitivities of the rpoH major promoters to DNA supercoiling and temperature shift indicate that these promoters are under differential control. Taken together with the in vivo enhancement by heat shock of rpoH P2 transcription, we propose that at least one of the mechanism(s) of induction of sigma 32 synthesis by heat shock is the activation of the rpoH P2 promoter by temperature up-shift.