The heat shock response of Escherichia coli

A large variety of stress conditions including physicochemical factors induce the synthesis of more than 20 heat shock proteins (HSPs). In E. coli, the heat shock response to temperature upshift from 30 to 42°C consists of the rapid induction of these HSPs, followed by an adaptation period where the rate of HSP synthesis decreases to reach a new steady-state level. Major HSPs are molecular chaperones, including DnaK, DnaJ and GrpE, and GroEL and GroES, and proteases. They constitute the two major chaperone systems of E. coli (15–20% of total protein at 46°C). They are important for cell survival, since they play a role in preventing aggregation and refolding proteins.The E. coli heat shock response is positively controlled at the transcriptional level by the product of the rpoH gene, the heat shock promoter-specific σ32 subunit of RNA polymerase. Because of its rapid turn-over, the cellular concentration of σ32 is very low under steady-state conditions (10–30 copies/cell at 30°C) and is limiting for heat shock gene transcription. The heat shock response is induced as a consequence of a rapid increase in σ32 levels and stimulation of σ32 activity. The shut off of the response occurs as a consequence of declining σ32 levels and inhibition of σ32 activity. Stress-dependent changes in heat shock gene expression are mediated by the antagonistic action of σ32 and negative modulators which act upon σ32. These modulators are the DnaK chaperone system which inactivate σ32 by direct association and mediate its degradation by proteases. Degradation of σ32 is mediated mainly by FtsH (HflB), an ATP-dependent metallo-protease associated with the inner membrane. There is increasing evidence that the sequestration of the DnaK chaperone system through binding to misfolded proteins is a direct determinant of the modulation of the heat shock genes expression. A central open question is the identity of the binding sites within σ32 for DnaK, DnaJ, FtsH and the RNA polymerase, and the functional interplay between these sites. We have studied the role of two distinct regions of σ32 in its activity and stability control: region C and the C-terminal part. Both regions are involved in RNA polymerase binding.