A recombination function essential to the growth of bacteriophage P22.

Mutants of the Sahrwneliu phage P22 have been isolated which cannot grow after infection of recombination-deficient (ret-) hosts. These mutants (designated erffor essential recombination function) are defective in a phage-specified recombination system, as shown by twoand three-factor crosses carried out in WC + and rethosts. Four erf mutants examined in detail fall into one complementation group ; erf+ is dominant to erf -. The erf gene is located in the “early” region of the phage genetic map. The phenotype of erfmumnts after infection of rethosts includes failurn to grow, to lysogenize, and to sustain phage DNA synthesis beyond about one round of replication. Late phago functions appear to be expressed normally by erf mutants. Circularization of the infecting circularly permuted, t,erminally repetitious phage DNA seems to be the essential step for which recombination is required. This idea is supported by the observation that an erf phage can grow normally in retcytoplasm after induction of a lysogen ; in this case the DXA is presumably circularized by the prophage excision mechanism. We conclude that recombination is essential to growth because the linear infecting DNA must be circularized by recombination. The inability to sustain DNA synthesis in the absence of recombination suggosts that circularization is topologically essential to successful replication of the phage genome.