Microcin E492 is a polypeptide antibiotic that is produced and excreted by Klebsiella pneumoniae RYC492. The genetic determinants for microcin synthesis and immunity were cloned in Escherichia coli VCS257 into the cosmid vector pHC79, starting from total DNA of K. pneumoniae RYC492. The microcin E492 expressed in E. coli had the same properties as that of K. pneumoniae, i.e., the same molecular weight, the ability to form ionic channels in planar phospholipid bilayers, and essentially identical biological properties. Microcin E492 expression in E. coli, like that in K. pneumoniae, was mainly in the exponential phase of growth, declining in the stationary phase. The immunity determinant was subcloned into the same vector, and its expression was found to disappear in the stationary phase. This phenomenon is not dependent on rpoS, the stationary-phase sigma factor.Microcins are a family of low-molecular-weight antibiotics produced by members of the family Enterobacteriaceae. Microcins, unlike colicins, are not induced by DNA-damaging agents, but like for colicins, bacterial strains producing a specific microcin are immune to the same microcin (3).Microcin E492 is a polypeptide with an M r of 6,000 (14) that is produced by Klebsiella pneumoniae RYC492 and is active on strains of Escherichia coli, Klebsiella, Salmonella, Citrobacter, Enterobacter, and Erwinia (6). The mechanism of action is through membrane depolarization (9) induced by the formation of pores in the bacterial membrane (14). Studies on channel-forming bacteriocins produced by gram-negative bacteria have been reported only for colicins, all of them of high molecular weight (reviewed in reference 5). The other described bacteriocin which has the cytoplasmic membrane as target is colicin V. This bacteriocin has been reclassified as a microcin because it has a molecular weight of 6,000 and does not have most of the properties which are associated with colicins (10). Despite the fact that microcin E492 has many feature in common with colicin V, they are not closely related, because ColV ϩ strains that have high-level immunity to colicin V are fully susceptible to microcin E492 (9). Those colicins which act by depolarizing the bacterial energy-transducing membrane in vivo are able to form aqueous channels in both closed liposomes and planar membranes. However, despite the fact that colicin V inhibits the capacity of E. coli to carry out active transport of proline and to generate a membrane potential (26), it has not been possible to find an effect of colicin V on asolectin liposomes, and it remains to be established whether this difference reflects a difference in the in vivo mode of action. Thus, so far the only microcin described to be a channel-forming bacteriocin is microcin E492 (14). Nevertheless, low-molecular-weight pore-forming bacteriocins of gram-positive bacteria have been described, among them the lantibiotic nisin and lactococcins A and B, which are nonlantibiotic heatstable bacteriocins (reviewed in references 1, 13, and 24). The former has...