Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    adsorb the colicin, but which are not killed by it, are said to be tolerant; these are defective in some stage of killing subsequent to adsorption. Acquisition of the plasmid factor responsible for synthesis of a colicin confers immunity to it; colicinogenic cells adsorb colicin as readily as noncolicinogenic cells, but the killing action is inhibited. (Note that im munity to colicin killing should not be confused with the superinfection immunity that governs the ability of plasmids to survive in a cell line.) Immunity has been used to distinguish between different colicins that use the same receptor; for example, two colicins were originally classified as the I type by their common adsorption, but the presence of each factor confers immunity only to its own colicin, thus defining colicins la and lb. Similarly colicins E2 and E3 share a receptor but differ in immunity. The terms la and E2, for example, therefore describe the colicin protein itself, that is, the killing agent. The nucleic acid plasmids which carry the information to specify these colicins are described as the colicin factors Col la and Col E2. It addition to describing adsorption group and immun ity, it may be useful to indicate the origin of a colicin factor; thus Col E2-P9 indicates the Col E2 factor derived from Shigella sonnei strain P9 (for review see Nomura, 1967a). The naming of the E group colicins is anomalous because some bacterial mutants were obtained that were resis tant to all three E colicins; however, Hill and Holland (1967) identified mutants resistant only to E2 and E3 and thus showed that El must utilize a different receptor (although it might share a component with the E2/E3 receptor). In a colicinogenic bacterial population, only a small proportion of the cells produces colicin. Ozeki, Stockerand De Margerie (1959) developed two techniques to follow the release of colicin E2 by individual bacteria. In one series of experiments, single colicinogenic bacteria produced small clear spots in the growth of a sensitive strain; since these spots contained of the order of 6000 bacteria, at least this number of colicin molecules must be released in a single event. Another set of experiments assayed the bacteriacidal action of products released into microdrops of broth by single colicinogenic bacteria; these experiments showed that the cells releasing colicin are inviable. Only a small (random) fraction of the colicinogenic bacteria manufacture the killing colicin; and these cells thus are killed in the process. Several colicins have been prepared in purified form. In each case the biologically active agent is a protein. Colicins which are related in their use of the same receptor generally appear to take a similar physicochemi- cal form (although their modes of killing may be quite different). Konisky (1972) found that both the la and lb colicins are proteins of about 80,000 daltons; they have the same amino acid composition, isoelectric point,