The Role of Helper Phage in gal Transduction

Abstract

Lysogeny by phage λ involves integration of the viral DNA into the host chromosome, as suggested by Campbell (1962). Integration occurs through site-specific recombination (Weil and Signer, 1968; Echols et al., 1968), catalyzed by the protein product of the λ int gene (Zissler, 1967; Gingery and Echols, 1967; Gottesman and Yarmolinsky, 1968a) (Fig. 1). When the prophage is induced, the viral DNA comes out of the host chromosome by another site-specific recombination, catalyzed by the protein products of the int and xis genes (Gottesman and Yarmolinsky, 1968b; Gingery and Echols, 1968; Kaiser and Masuda, 1970; Echols, 1970). In terms of the nomenclature for “attachment regions” of Fig. 1, the integrative recombination involves PP′+BB′→intBP′+PB′; the excisive recombination involves BP′+PB′→xisintBB′+PP′.

Phage λ is able to carry out transduction of the host gal genes near the λ prophage site (Morse et al., 1956) through occasional production of λgal transducing phage. The DNA of the λgal transducing phage contains both viral and host genes and is presumed to have its genesis in an aberrant excision event which recombines a portion of the phage DNA with host DNA (Campbell, 1962) (see Fig. 1). Although the typical λgal transducing phage retains the genetic region specifying the regulatory, replication, and integration proteins and thus has no obvious functional defect likely to affect integration, transduction by λgal is much rarer than lysogeny by normal λ (Campbell, 1957; Arber et al., 1957). Transduction frequencies become comparable to normal lysogeny frequencies when transduction is carried out by mixed infection with...