Updated project metadata. The outer membrane (OM) of Gram-negative bacteria confers innate resistance to toxins and antibiotics. Integral -barrel outer membrane proteins (OMPs) are assembled into the OM by the beta-barrel assembly machine (BAM), which is composed of one OMP, BamA, and four lipoproteins, BamBCDE. BamBCE can be removed individually with only minor effects on barrier function. However, depletion of BamA or BamD, the only BAM components conserved in all diderm bacteria, causes a global defect in OMP assembly and results in cell death. We have identified a gain-of-function mutation, bamAE470K, that bypasses the requirement for BamD. Although bamD::kan bamAE470K cells exhibit growth defects, they assemble OMPs with surprising robustness. Our results demonstrate that the reactions required for beta-barrel folding and membrane integration are catalyzed solely by BamA.