CbrA is a flavin adenine dinucleotide protein that modifies the Escherichia coli outer membrane and confers specific resistance to Colicin M.

Colicin M (Cma) is a protein toxin produced by Escherichia coli that kills sensitive E. coli cells by inhibiting murein biosynthesis in the periplasm. Recombinant plasmids carrying cbrA (formerly yidS) strongly increased resistance of cells to Cma, whereas deletion of cbrA increased Cma sensitivity. Transcription of cbrA is positively controlled by the two-component CreBC system. A ΔcreB mutant was highly Cma sensitive because little CbrA was synthesized. Treatment of CbrA-overproducing cells by osmotic shock failed to render cells Cma sensitive because the cells were resistant to osmotic shock. In a natural environment with a growth-limiting nutrient supply, cells producing CbrA defend themselves against colicin M synthesized by competing cells. Isolated CbrA is a protein with noncovalently bound flavin adenine dinucleotide. Sequence comparison and structure prediction assign the closest relative of CbrA with a known crystal structure as digeranylgeranyl-glycerophospholipid reductase of Thermoplasma acidophilum. CbrA is found in Escherichia coli, Citrobacter, and Salmonella bongori but not in other enterobacteria. The next homologs with the highest identity (over 50%) are found in the anaerobic Clostridium botulinum group 1 and a few other Firmicutes.