Porin biogenesis activates the σE response in Salmonella hfq mutants

In Salmonella enterica, loss of RNA chaperon Hfq promotes proteolytic cleavage of anti-σE factor RseA leading to the constitutive induction of the σE-dependent envelope stress response. Seeking to identify the source of the inducing signal, in the present study we measured RseA cleavage and σE-dependent transcription in strains lacking relevant outer membrane protein (omp) genes. We found removal of the main Salmonella porin, OmpD, to markedly reduce σE activation in hfq mutant cells. Subsequent removal of LamB and of OmpC further attenuated the response, indicating that different OMPs collectively contribute to the σE-activated phenotype. Thus, loss of Hfq-mediated regulation might cause unfolded OMPs to accumulate in the periplasm, triggering the σE response. These findings corroborate the role of Hfq protein as a pleiotropic regulator of OMP biogenesis in Gram-negative bacteria.