Colicin B: mode of action and inhibition by enterochelin

Adsorption of colicin B to a sensitive strain of Escherichia coli results in rapid cessation of deoxyribonucleic acid, ribonucleic acid, and protein synthesis. Some classes of mutants insensitive to colicin B hyperexcrete a colicin inhibitor into their growth medium. This inhibitor functions by preventing adsorption of colicin B and does not rescue cells to which colicin has already adsorbed. The inhibitor is insensitive to nucleases, proteolytic enzymes, and lysozyme and is not extracted into organic solvents. The inhibitory material has a low molecular weight, which rules out identification as lipopolysaccharide, although purified lipopolysaccharide has some inhibitory activity. Evidence is presented that the inhibitor is enterochelin, an iron chelator which is the cyclic trimer of 2,3-dihydroxybenzoylserine. Enterochelin does not inhibit colicin M, a colicin that is produced by many strains colicinogenic for colicin B.