Regulation of the promoters and transcripts of rpoH, the Escherichia coli heat shock regulatory gene.

In Escherichia coli the product of the rpoH (htpR) gene, sigma 32, directs RNA polymerase to initiate transcription from heat shock promoters at all temperatures. Transcription of the heat shock genes is increased when cells are exposed to high temperatures because of increased transcription initiation by sigma 32-RNA polymerase. As a step toward understanding the regulation of the heat shock response we have examined the transcription of the rpoH gene. Using S1 mapping, promoter cloning, and in vitro transcription, we have identified the promoters and the terminator for the rpoH transcription unit. The rpoH transcripts are monocistronic and originate from at least three promoters. None of the promoters is recognized by sigma 32-RNA polymerase. Two are recognized by sigma 70-RNA polymerase and are active at both low and high growth temperatures. We do not know what form of RNA polymerase recognizes the third promoter. Transcripts from this promoter are abundant only at high temperature and are present after shift to the lethal temperature of 50 degrees C, even at times when there are no detectable transcripts from the other rpoH promoters. The amount of rpoH mRNA increases fivefold by 8 min after shift from 30 to 43.5 degrees C but rpoH mRNA synthesis increases by less than twofold, indicating that there is post-transcriptional control of the level of rpoH mRNA and presumably of sigma 32.