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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    Plasmids able to transfer infectiously between cells; these may also trans fer chromosome markers (see Ozeki and Howarth, 1961; Clowes, 1961). These factors are of the same order of size as the sex factors and // + R factors and possess either the same or analogous transfer systems. The small Col E factors, however, are not transmissible and pass between cells only when conjugation is sponsored by some other factor. This distinction was first drawn by Ozeki, Stocker and Smith (1962), who identified determinants for five colicins able to transfer in mixed cultures, in contrast with the inability of Col E or Col K plasmids to do so (for review see Hardy, 1975). Col I factors are self repressed, like the R factors. Stocker, Smith and Ozeki (1963) found that when a Col I broth culture of S.typhimurium LT2 is incubated with a noncolicinogenic acceptor strain, < 0.1% of the recip ient bacteria become colicinogenic during a 1 hour incubation; but after 18 hours about 50% of the recipient cells carry Col I. Only a small proportion of the recipients acquire col I in the brief transfer period because a very small proportion, about 0.02% of the colicinogenic bacteria, are competent donors. Longer periods of incubation allow infectious spread of col- icinogeny because newly colicinogenic cells are good donors. The high level of competence in donors that have recently acquired Col I persists for 2-7 generations; in these HFC (high frequency colicinogeny transfer ring) cultures, 30-100% of the cells are competent donors. Monk and Clowes ( 1964a,b) reported that Col I conjugation is independent of that promoted by the sex factor; and the efficiency of the self-repression system varies with the bacterial host, being less stringent in E.coli (about 1% competent donors in an established culture) than in S.typhimurium (only 0.02%). Strains of S.typhimurium LT2 carrying Col E2 alone are not able to transfer it. However, Smith, Ozeki and Stocker (1963) found that HFC cultures of Col I can transmit Col El and Col E2. In crosses between an Hfr Col El strain and a noncolicinogenic F factor, Clowes (1963) found no relationship between transfer of Col El and any chromosome position; the factor therefore appears to be extrachromosomal and must take ad vantage of the formation of mating pairs to transfer between bacteria. In experiments of this sort, all of the colicin factors Col El, Col E2, Col I, Col V, appeared to be extrachromosomal since their transfer showed no correlation with any chromosome marker (Nagel de Zwaig, Anton and Puig, 1962; Nagel de Zwaig and Puig, 1964). A relationship between Col V factors and the sex factor was implied by the observations of Kahn and Helinski (1964, 1965) that Col V cells are susceptible to ¿-specific RNA phages and so must produce F-type pili. Further, Col V and F are mutually exclusive; and Col V promotes transfer