Characterization of a Novel RNA Polymerase Mutant of Escherichia coli That Confers Thermotolerance and Chaperone Independence

Bacterial cells activate stress-sensing and stress-mitigating pathways by employing a number of transcription regulators, some of which can bind directly to RNA polymerase to activate stress-specific response pathways. However, mutations in the RNA polymerase genes can accumulate under certain selection conditions and activate stress-mitigating pathways in a manner that is partly independent of pathway-specific regulators. In this study, we characterized a novel mutation in the rpoB gene that transforms RNA polymerase into a "stringent" polymerase in the absence of one of the key stringent response (SR) activating factors (p)ppGpp, produced by the relA gene product. The mutant RNA polymerase allele, rpoB58, elevated thermotolerance and permitted growth without the key molecular chaperones (DnaKJ) and proteases (Lon, ClpP) at temperatures nonpermissive to cells expressing the wild type RNA polymerase genes. Remarkably, rpoB58 also reversed the cell division defect of ΔdnaJ at a nonpermissive temperature but could not overcome its lambda phage-resistant phenotype. The rpoB58-mediated rescue of the ΔdnaKJ growth defect was partly reversed in the absence of DksA, a protein that acts synergistically with (p)ppGpp to transform RNA polymerase into a stringent state. The data suggest that pre-activated SR confers thermotolerance and chaperone independence in part by lowering protein synthesis.