Lon Protease is Needed to Resume Growth of Escherichia coli rpoH Mutant Following Gamma Radiation

Heat shock proteins (HSPs) such as chaperones and proteases are upregulated to ensure correct protein folding and to prevent the formation of toxic protein aggregates in response to heat shock or other insults that increase cellular protein misfolding. In Escherichia coli, which has been used as a model organism in research in microbial genetics and physiology, HSPs are induced by temperature upshift from 30℃ to 42℃, and their induction is regulated by the alternative sigma factor, σ³², encoded by the rpoH gene. The observation that post-irradiation incubation of E. coli at temperatures higher than 37°C resulted in increased survival rates has led to the proposal that HSPs are required to survive stress from ionizing radiation. We have investigated this hypothesis by examining the effects of rpoH mutation on cell survival and postirradiation recovery. The wild-type E. coli strain (WT) exposed to gamma radiation (γ-radiation) exhibited a clear decrease in survival relative to non-irradiated counterparts, but there was no significant difference in the survival outcomes between WT and an rpoH mutant strain (ΔrpoH). When cells irradiated with doses of 1, 2, and 3 kGy were re-inoculated into fresh media, the lag phase of ΔrpoH became longer than that of WT in a dose-dependent manner. Expression of the heat shock protease Lon could partially restore the growth defect observed in irradiated ΔrpoH. These findings suggest that Lon may play a role in the post-irradiation recovery period by degrading its primary substrate SulA, which inhibits cell division