Infectious multiple drug resistance / S. Falkow.

  • Stanley Falkow
Date:
1975
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Infectious multiple drug resistance / S. Falkow. Source: Wellcome Collection.

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    16 Chapter 2 гаге lac* colonies which differed from usual recombinant populations in several respects. Ordinarily one thinks of a bacterial recombinant cell as one in which the donor gene has replaced its recipient counterpart by recombination. These rare lac* recombinant colonies, however, were unstable and constantly segregated lac~ cells at a frequency of about 10^ per cell per generation. They appeared, therefore, to be relatively stable partial diploids for the lac* region. The lac* cells were also intermediate donors of the chromosome and, in fact, transferred the chromosome with the same polarity as the original Hfr strain. The sex factor and lac* genes, however, were transferred together with the very high frequency of an autonomous F agent. In short, F and the lac* genes appeared to behave as a single unit of replication and transfer! Hybrid or substituted sex factors such as Fj and V-lac* are called F'-factors (F-prime factors), and the general process whereby the sex factor acquires host genetic material has been given the name sexduction or F-duction. Since 1959-1960, when this process was first described, a large number of independently isolated F'-factors have been derived from different Hfr strains. Some F'-factors are known which carry over 10% of the entire E. coli chromosome. I should stress that sexduction makes it possible to isolate virtually any gene on an F-factor and, by virtue of its associated infectivity, to create bacterial strains which are diploid for virtually any desired gene. These strains have been of enormous value in establishing the dominance and recessiveness of different alleles of the same gene and have proved especially valuable in studying the regulatory interactions of genes. F'-factors probably arise by (1) a reciprocal crossover between chromosomal sites on either side of an integrated sex factor, or (2) a reciprocal crossover between an adjacent chromosomal site and a site in the integrated F. The products of the first type of exchange would contain both an early chromosomal gene and a terminal chromosomal gene of the ancestral Hfr chromosome (figure 2.4: type II F' excision). Such an F'-factor would contain the entire F genome plus the acquired chromosomal genes and therefore would always be physically larger than the autonomous wild-type F. Interestingly enough the bacterial cell in which this event occurred would suffer a deletion in its chromosome corresponding to the material present in the F'-factor. The products of the second type of exchange would contain either a late gene or an early gene (figure 2.4: type I F'excision). Moreover since the crossover would occur within F it is possible that a piece of F would be left behind in the chromosome, as was the case for F2. Also the host chromosome would suffer a deletion of the material present in the F'-factor. Both general types of F'-factors have, in fact, been isolated, and it has been confirmed that the host chromosome remaining after F' formation does contain a deletion corresponding to the material now residing with F in the autonomous state. The deletion does not result in a broken chromosome,
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