In Escherichia coli, the absence of the major periplasmic protease DegP together with an outer membrane protein (OMP) assembly factor, BamB, causes acute envelope stress and confers a temperature-sensitive phenotype. Characterization of suppressors that overcome the temperature sensitivity of a degP bamB strain revealed a novel protein, MzrA, pleiotropic envZ mutations, and mutations that disrupt the rseA gene. Genetic evidence reveals that MzrA functions through the EnvZ/OmpR two-component system to modulate its output but not its ability to sense and respond to various environmental cues. MzrA is localized to the inner membrane with the N-terminus in the cytoplasm and C-terminus in the periplasm and interacts directly with EnvZ. The overlapping phenotypes of pleiotropic envZ mutants to strains over-expressing MzrA indicate MzrA either enhances EnvZ's kinase or inhibits its phosphatase activity to elevate cellular levels of OmpR∼P. MzrA is shown to link the CpxA/CpxR and EnvZ/OmpR two-component systems and is feedback inhibited by OmpR∼P. Deletion and domain replacement analysis narrowed the functional section of MzrA to residues 32-104, of which amino acids D51 and I78 play important roles in the interaction with EnvZ.

The MzrA- or EnvZ-dependent suppression of the temperature sensitive phenotype of a degP bamB strain is in part due to a down-regulation of major β-barrel OMPs such as OmpF and LamB. Consistent with this, mutations in rseA activate the σ^E stress response system which in part alleviates envelope stress by inhibiting synthesis of β-barrel OMPs by small RNAs. Direct removal of β-barrel OMPs also overcomes the temperature sensitivity of degP or degP bamB strains. Thus, the removal of β-barrel OMPs is a major mechanism of relieving stress in the envelope. Conditional synthetic phenotypes of strains with defective β-barrel assembly machinery (Bam) lacking the CpxR response regulator reveal an important role for the CpxAR two-component system in the response to misfolded β-barrel OMPs, primarily because of the ability of CpxR∼P to constitutively activate, and up-regulate DegP in response to aberrant OMP assembly.

Finally, global transcriptome analysis of a strain expressing a pleiotropic EnvZ mutant, EnvZ_[R397L], reveal a novel role of the EnvZ/OmpR system in the maintenance of cellular iron homeostasis.