The Gain-Of-Function Allele Bama(E470k) Bypasses The Essential Requirement For Bamd In Beta-Barrel Outer Membrane Protein Assembly

The outer membrane (OM) of gram-negative bacteria confers innate resistance to toxins and antibiotics. Integral beta-barrel outer membrane proteins (OMPs) function to establish and maintain the selective permeability of the OM. OMPs are assembled into the OM by the beta-barrel assembly machine (BAM), which is composed of one OMP-BamA-and four lipoproteins-BamB, C, D, and E. BamB, C, and E can be removed individually with only minor effects on barrier function; however, depletion of either BamA or BamD causes a global defect in OMP assembly and results in cell death. We have identified a gain-of-function mutation, bamA(E470K), that bypasses the requirement for BamD. Although bamD::kan bamA(E470K) cells exhibit growth and OM barrier defects, they assemble OMPs with surprising robustness. Our results demonstrate that BamD does not play a catalytic role in OMP assembly, but rather functions to regulate the activity of BamA.