Hfr Strains of Escherichia coli K-12
نویسنده
چکیده
Since the first reports by Cavalli-Sforza (9) and Hayes (24) of strains of Escherichia coli K-12 which are conjugational donors of chromosomal markers at high frequency (Hfr strains), many other Hfr strains have been isolated and our understanding of the mechanisms involved in Hfr formation has increased considerably. All Hfr strains arise from the integration of a conjugative plasmid into the bacterial chromosome by one of several possible types of recombination events. The most commonly used Hfr strains have been formed by either spontaneous or UV-induced integration of the E. coli F factor (see chapter 126). F integration can also be selected for at high temperature by the use of E. coli mutants which are defective in initiation of DNA replication at high temperature (28, 41, 47). The wild-type F factor has been found to integrate at at least 20 different sites on the E. coli K-12 chromosome (37); however, these sites are very nonrandom, and the spectrum of possible sites may vary from strain to strain (13). Different investigators have independently isolated Hfr strains with similar, if not identical, insertion sites (points of origin). As reviewed by Davidson, Deonier, Ohtsubo, and others (16, 18, 27, 48, 49; see chapters 111 and 129), the repeated integration of F at certain sites of the chromosome has been shown in at least some cases to be due to recombination events between an insertion sequence (IS element) on the F factor and a homologous IS element on the chromosome. Some of the chromosomal IS elements probably involved in the formation of certain Hfrs are indicated in Fig. 1. Note that for some Hfrs, there is no known IS element at a chromosomal position near the observed point of origin. The mechanism of F integration at these sites is not clear. Both integration and excision of F at IS sequences are considerably reduced in recA mutants (12, 19), but this is not true in the case of certain R factors (25, 26). In some cases integration has involved recombination between a larger transposable element, γδ (= Tn1000; see chapters 124, 126, and 140), and an integrated copy of γδ in the E. coli chromosome, which does not normally carry γδ (22, 34).
منابع مشابه
Conjugation in Escherichia coli.
Boyer, Herbert (Yale University, New Haven, Conn.). Conjugation in Escherichia coli. J. Bacteriol. 91:1767-1772. 1966.-The sex factor of Escherichia coli K-12 was introduced into an E. coli B/r strain by circumventing the host-controlled modification and restriction incompatibilities known to exist between these closely related strains. The sexual properties of the constructed F(+) B strain and...
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