A chaperone network controls the heat shock response in E-coli

The heat shock response controls levels of chaperones and proteases to ensure a proper cellular environment for protein folding. In Escherichia coli, this response is mediated by the bacterial-specific transcription factor, sigma(32). The DnaK chaperone machine regulates both the amount and activity of sigma(32), thereby coupling sigma(32) function to the cellular protein folding state. In this manuscript, we analyze the ability of other major chaperones in E. coli to regulate sigma(32), and we demonstrate that the GroEL/S chaperonin is an additional regulator of sigma(32). We show that increasing the level of GroEL/S leads to a decrease in sigma(32) activity in vivo and this effect can be eliminated by co-overexpression of a GroEL/S-specific substrate. We also show that depletion of GroEL/S in vivo leads to up-regulation of sigma(32) by increasing the level of sigma(32). In addition, we show that changing the levels of GroEL/S during stress conditions leads to measurable changes in the heat shock response. Using purified proteins, we show that that GroEL binds to sigma(32) and decreases sigma(32)-dependent transcription in vitro, suggesting that this regulation is direct. We discuss why using a chaperone network to regulate sigma(32) results in a more sensitive and accurate detection of the protein folding environment.