Involvement and necessity of the Cpx regulon in the event of aberrant beta-barrel outer membrane protein assembly.

The Cpx and sigma(E) regulons help maintain outer membrane integrity; the Cpx pathway monitors the biogenesis of cell surface structures, such as pili, while the sigma(E) pathway monitors the biogenesis of beta-barrel outer membrane proteins (OMPs). In this study we revealed the importance of the Cpx regulon in the event of beta-barrel OMP mis-assembly, by utilizing mutants expressing either a defective beta-barrel OMP assembly machinery (Bam) or assembly defective beta-barrel OMPs. Analysis of specific mRNAs showed that Delta cpxR bam double mutants failed to induce degP expression beyond the wild type level, despite activation of the sigma(E) pathway. The synthetic conditional lethal phenotype of Delta cpxR in mutant Bam or beta-barrel OMP backgrounds was reversed by wild type DegP expressed from a heterologous plasmid promoter. Consistent with the involvement of the Cpx regulon in the event of aberrant beta-barrel OMP assembly, the expression of cpxP, the archetypal member of the cpx regulon, was upregulated in defective Bam backgrounds or in cells expressing a single assembly-defective beta-barrel OMP species. Together, these results showed that both the Cpx and sigma(E) regulons are required to reduce envelope stress caused by aberrant beta-barrel OMP assembly, with the Cpx regulon principally contributing by controlling degP expression.