Genetics of bacteriophage P22. II. Gene order and gene function.

Although large numbers of temperate phages are known to exist in nature, only a small number of these have been examined in much detail. Foremost among these is the coliphage ~, which has been studied in great depth over the past two decades. However, it is already quite clear that phage ~ represents only one alternative among many with respect to any of its interesting properties. Most other temperate phages which have been studied differ in essential aspects [see Calendar (1970) for review]. The temperate Salmonella phage P22 differs in many ways from phage ),. The DNA of X has cohesive ends (Hershey and Burgi, 1965), whereas phage P22 has linear DNA with circularly permuted sequence and terminal repetitions (Rhoades et al., 1968). X grows in the absence of recombination (Signer and Weil, 1968), but P22 requires recombination for growth (Botstein and Matz, 1970). X carries out specialized transduetion only, whereas P22 carries out both generalized and specialized transduction (Wing, 1968). X prophage is repressed completely by a single represser (Ptaslme and Hopkins, 1968; Hopkins and Ptashne, 1971) which completely specifies immunity. P22, on the other hand, uses two gene products to repress the prophage (Gough, 1968) and two separate genetic elements contribute to the specificity of immunity (Bezdek and