Mode of action of colicin E2, colicin E3 and cloacin DF13.

The bacteriocins colicin E2, colicin E3 and cloacin DF13 are bactericidal proteins which are excreted by producing cells as a complex of two polypeptides, present in equimolar amounts. The high-molecular weight component is responsible for the biochemical lesion induced by these bacteriocins. The low-molecular weight component, the so-called immunity protein, was shown to act as inhibitor of the high-molecular weight component. Colicin E2, freed from immunity protein, causes endonucleolytic degradation of DNA. Colicin E3 and cloacin DF13, freed from immunity protein, induce ribosome inactivation by endoribonucleolytic cleavage of 16S rRNA. A possible role of the immunity protein in the killing activity of the bacteriocins is not quite clear. Probably it keeps the high-molecular weight component in the right configuration for penetration of the cell wall layers. Studies on structure-function relationships have shown that cloacin DF13 has a domain-type structure. Various properties have been located in the hydrophilic C-terminal part of the molecule.