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.

    119/952 (page 99)
    been encouraged by the isolation of mutations which render episomes unable to integrate into the bacterial chromosome, so that strictly speak ing the mutant should be described as a plasmid rather than episome. Excision of F and Formation of F' Factors The distinction between Hfr and F + bacteria is clear. Although both possess an F factor, produce F pili, and can act as donors for host markers, the differences in the state of the sex factor mean that the Hfr transfers the host chromosome at high frequency with a specific orienta tion whereas the F^ transfers the sex factor efficiently but transfers chromosomal markers less frequently. Another type of donor was first isolated by Adelberg and Burns (1960), who were working with the P4x Hfr strain, which transfers pro, leu, thr as early markers and lac as a late marker (the sequence shown for Hfr2 in Figures 2.9 and 2.10). The progeny of P4x Hfr x F~ matings are F, except when they are selected for the distal donor marker lac + —the lac + recombinants are usually Hfr type because the sex factor appears to be transferred as a distal marker and thus is closely linked to the lac locus. A subclone of this strain, P4x-1, however, had the unusual property that recombinants gaining early chro mosome markers acquired also the capacity to behave as genetic donors. When used as donors, these recombinants transfer the host chromosome with the same orientation as the parent Hfr strain, although at a somewhat lower frequency. The ability of recombinants gaining early markers also to act as donors is perpetuated through a series of crosses with F recipients. While chromosome transfer by these cells resembles that of the ancestral Hfr strain, the sex factor appears to be independently and rapidly transferred in the manner characteristic of F* donors. This strain thus appears to carry a modified sex factor, termed F', with characteristics intermediate between Hfr and F+ types; chromosome transfer has a specific orientation but sex factor transfer is infectious. A model to account for these features is shown in Figure 2.13; this supposes that an illegitimate genetic exchange took place when the F' factor of the P4x-1 strain was excised from the chromosome. Instead of reversing integration as shown in Figure 2.12 by recombination between the original sites, a genetic exchange might take place between a site located within the sex factor and a site to one side of it on the chromosome. This type I exchange generates an F' factor which carries certain chromosomal genes and lacks some of the material of the F factor; and it leaves a bacterial chromosome which has gained some of the material of the sex factor but has lost certain genes. In the exchange shown in Figure 2.13, the F' factor carries the distal chromosomal gene z.