Characterization of the bamA101 Mutation in the Context of the Rcs Envelope Stress Response

Abstract

Gram-negative bacteria have an outer membrane (OM) that acts as an essential permeability barrier. The OM is an asymmetric bilayer that contains two major classes of proteins: peripheral lipoproteins and integral β-barrel proteins referred to as OMPs. OMPs are assembled by the Bam complex, which consists of the β-barrel BamA and the lipoproteins BamB, -C, -D, and -E. bamA101 is a mutation that causes a decrease in the levels of BamA, and despite causing only mild OMP assembly defects, results in an OM barrier defect distinct from other Bam mutants. In addition, this mutation causes an unusually high amount of Rcs signaling, which is a stress response sensitive to OM defects. In order to understand how bamA101 is inducing Rcs and causing the OM defect, we isolated and characterized four suppressor mutants. These suppressor mutations were found to reduce Rcs signaling, but did not restore OMP assembly defects. In addition, the σE response, another envelope stress response, was still induced in most suppressors, and the growth defect of bamA101 was not corrected. However, the suppresor mutations were able to restore barrier function of the OM, which suggests a model of Rcs signaling in the bamA101 mutant where the sensor, RcsF, is able to directly detect OM perturbations.. Although the genes harboring these mutations were not found, the loci that conain these mutations were identified by transduction mapping. Further sequencing of genes in these regions will hopefully ascertain the identity of these mutations and provide further insight into the mechanism of suppression.