Characterisation of a phoP P1vir transduction defect and the implication for the regulation of the extracytoplasmic stress response in Escherichia coli

The sensor kinase PhoQ and its cognate response regulator PhoP constitute a two-component system, which is primarily responsible for sensing and responding to Mg2+ starvation in Escherichia coli. Additionally, there is growing evidence of regulatory links between PhoPQ and constituents of the outer membrane. Furthermore, it has been shown that PhoPQ is regulated negatively by MicA, an sRNA controlled by sigmaE. Encoded by rpoE, sigmaE is an alternative sigma factor that is activated in response to extracytoplasmic stress, specifically misfolded outer membrane proteins. Surprisingly, it was not possible to generate ?phoP mutants, using P1vir transduction under standard conditions and kanamycin as the selective agent. Furthermore, a statistical analysis of these results indicates they cannot be explained by chance alone. The results show that PhoP is required for sigmaE activity in an RseA-independent manner, thereby suggesting that PhoP is a chief regulator of sigmaE activity. It is likely that diminished sigmaE activity in a phoP mutant, extracytoplasmic stress and OM deformation, caused by the reagents used in P1vir transduction, are responsible for the inability to transduce the phoP allele. Finally, evidence has been found relating to a second mechanism through which PhoP directly represses rpoE expression, thereby introducing further complexity into the regulator relationship that exists between sigmaE and PhoP.