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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    specifies a pilus: pili can be classified into several groups, and the mem bers of each group are related but not necessarily identical. This implies that there may be polymorphism in the genes responsible for specifying each type of pilus. Mutants of the F-like fi + factors that are defective in their repression of the transfer system have been isolated in two ways. The R100-1 i~ mutant was selected by loss of its ability to repress the F factor (see above). Another technique for isolating such mutants was developed by Meynell and Datta (1967), who selected directly for fi + R factor variants able to express their own conjugation functions at high frequency. This cannot be done either by screening simply for increased transfer of chromosome markers (because the frequencies of transfer are too low) or by detecting increased transfer of R itself (because the frequencies are too great). They therefore used the $3 ( sfa + ) strain as host, enabling both R and F factors to promote chromosome transfer with increased frequency, in particular showing an increase in the frequency of transfer of the met + locus near the sfa site. The experimental protocol was to infect 93 cells with a wild type R factor, mutagenize with nitrosoguanidine or ultraviolet irradiation, and isolate clones that transfer met + to recipients. One possible flaw in this procedure is that the 93 cells might possess a defective F factor, so that the mutant R factors isolated for their ability to transfer met + might in fact comprise recombinants with this defective factor. Another procedure also therefore was introduced to isolate mutants: the use of microcolonies reduced the frequency of transfer of R to a more manageable level, so that it was possible to isolate mutants transferring the drug resistance markers at increased frequencies. The properties of the variants isolated by both procedures suggested that in fact both represent mutants of the R factor. All the mutants increased both the frequency of transfer of the R factor and the frequency of transfer of chromosome markers by about 300-fold; all displayed increased susceptibility to plaque formation by ^-specific phages, implying that F-like pili were produced. These mutants are de scribed as drd (for derepressed). Both the R100-1 (/) and various R drd mutants were used by Cooke and Meynell (1969) to examine the transfer of chromosome markers. In all cases recombinants were generated at frequencies similar to those pro duced by F f cells. This suggests an analogy between R-mediated and F-mediated transfer of chromosome markers, with the lower frequency of transfer by the R factor being due to the repression of its activity in most of the cells that carry it. R100-1 displayed no preferred site of attachment; derepressed derivatives of seven R factors, somefi + and some fi~, showed the same property. In these and other experiments, it was not possible to obtain Hfr lines in which R factors became stably integrated into the host