Regulation of the Heat Shock Response in Escherichia Coli: History and Perspectives

The heat shock response mediated by transcription factor σ is a major stress response to cope with heat and other stresses in Escherichia coli. Although much attention has been paid to the role of highly conserved heat shock proteins such as chaperones and proteases in sustaining cellular protein homeostasis under stress, relatively little is known about the dynamic nature of underlying regulatory mechanisms. When cells are suddenly exposed to high temperature, synthesis of σ is rapidly induced by activated translation of rpoH mRNA, which encodes σ, through disruption of mRNA secondary structure. The increased synthesis of σ is accompanied by stabilization of σ, which is normally very unstable and rapidly degraded by the membrane-localized FtsH protease. It was recently found that σ must be localized to the inner membrane by the SRP-dependent pathway to work properly for regulation, but the roles played by membrane and other components of the cell remained unknown. Random transposon mutagenesis of the strongly deregulated I54N-σ mutant has now started to unravel the complex regulatory circuit, involving membrane protein(s), other cellular components or σ-interfering polypeptides, for dynamic fine-tuning of σ activity that could be of vital importance for cell survival.