Bypassing BamD essentiality by mutations in a non-essential substrate

The β-barrel assembly machinery (Bam) is essential for assembling all transmembrane β-barrel outer membrane proteins in gram-negative bacteria. The Bam complex consists of the central β-barrel protein BamA and accessory Bam lipoproteins, including the widely conserved and essential BamD. BamD is assumed to be an essential regulator of OMP assembly, as its absence causes a global defect in OMP biogenesis. Here, we challenge this view by demonstrating that BamD essentiality is both conditional and substrate specific. In <i>Escherichia coli</i>, its function can be bypassed by preventing BamA jamming by a single, non-essential substrate RcsF. Our findings suggest that BamD plays two distinct roles in the Bam complex. It prevents improper RcsF engagement that can jam BamA, and it kinetically enhances BamA-mediated OMP assembly. Contrary to prevailing models, we demonstrate that the second function in general OMP assembly is not essential. We report a genetic background in which each Bam lipoprotein is dispensable for viability, providing a powerful new system for investigating their functions in OMP assembly in the context of unmodified, wild-type BamA.IMPORTANCEThe β-barrel assembly machinery (Bam) complex assembles all outer membrane proteins (OMPs) and is conserved and essential across all gram-negative bacteria. While BamA is critical for the folding and insertion of OMPs into the outer membrane, BamD is also considered essential for OMP assembly because its loss leads to a global reduction in OMP levels and cell death. Our results show that BamD is important but not essential for general OMP assembly. In <i>Escherichia coli,</i> BamD's essentiality arises from its role in preventing a single challenging substrate from jamming BamA and indirectly abolishing OMP assembly. This work challenges long-standing assumptions about BamD's function, underscores the substrate-specific roles of accessory Bam components, and offers important new considerations for interpreting genetic studies of the Bam complex.