An RNA thermometer

The expression of a conserved set of heat shock proteins is induced when cells grown at low temperatures are shifted to higher temperatures. Heat shock proteins are molecular chaperones or proteases that act to fold, translocate, or degrade proteins that appear to be misfolded or denatured upon heat shock. The heat shock response has been the focus of much research, and how the temperature signal is sensed and transduced to the biosynthetic machinery has been studied extensively.

The ς³² (RpoH) alternative ς-factor, which is encoded by the rpoH gene, is a key regulator of the Escherichia coli heat shock response (for review, see Gross 1996; Missiakas et al. 1996). Upon a temperature shift from 30°C to 42°C, ς³² accumulates and directs RNA polymerase to the promoters of the heat shock genes (Fig. ). Earlier studies showed that both increased synthesis and stability lead to the increased levels of ς³². The activity of ς³² and its association with RNA polymerase are also modulated by heat shock. A great deal has been learned about the increased stability of ς³² in response to increased temperature. During normal growth the half-life of ς³² is ∼1 min; upon upshift the half-life is increased to ∼5 min. Interestingly, the heat shock proteins DnaK, DnaJ, GrpE and HflB, whose expression is regulated by ς³², function to destabilize ς³². These proteins interact with ς³², sequestering it away from RNA polymerase and targeting it for degradation. Misfolded proteins that accumulate after heat shock appear to titrate the DnaK, DnaJ, and GrpE chaperones and the HflB protease away from ς³². Therefore, the pool of misfolded proteins is thought to be one measure of elevated temperature in the cell. Increased ς³² synthesis was known …