Sigma 32 synthesis can regulate the synthesis of heat shock proteins in Escherichia coli

The Escherichia coli rpoH (htpR) gene product, sigma 32, is required for the normal expression of heat shock genes and for the heat shock response. We present experiments indicating a direct role for sigma 32 in controlling the heat shock response. Both the induction and decline in the synthesis of heat shock proteins can be controlled by changes in the rate of synthesis of sigma 32. Specifically, we show that: (1) sigma 32 is an unstable protein, degraded with a half-life of approximately 4 min; (2) increasing the rate of synthesis of sigma 32, by inducing expression from a Plac or Ptac-rpoH fusion, is sufficient to increase the rate of synthesis of heat shock proteins; (3) during the shut-off phase of the heat shock response synthesis of sigma 32 is repressed post-transcriptionally, and the dnaK756 mutation, which causes a defect in the shut-off phase, prevents the post-transcriptional repression of synthesis of sigma 32. These results serve as a basis for understanding the role of DnaK in the heat shock response, the regulation of sigma 32 synthesis, and the role of sigma 32 in controlling transcription of heat shock genes.