Physical characterisation of the “Rac prophage” in E. coli K12

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Abstract
We confirm the hypothesis of Low (1973) that many E. coli K 12 strains contain a prophage (the Rac prophage) located a few minutes clockwise of the trp operon on the genetic map. We have used restriction endonucleases and 32P-labelled probes to construct a physical map of this prophage. Some E. coli K 12 strains, including AB1157, have lost the entire prophage, apparently by a specific deletion. This is consistent with prophage excision by site-specific recombination. λ reverse (λrev) phages (Zissler et al., 1971) are recombination proficient derivatives of phage λ in which the phage recombination functions have been replaced by analogous functions (RecE) derived from the host chromosome (Gottesman et al., 1974; Gillen et al., 1977). Our data support the origin of λrev phages by recombination between λ and the Rac prophage following excision of the Rac prophage from the E. coli chromosome. Important experimental data are included in the Figure legends.