A recombination function essential to the growth of bacteriophage P22

Abstract

Mutants of the Salmonella phage P22 have been isolated which cannot grow after infection of recombination-deficient ( rec −) hosts. These mutants (designated erf − for essential recombination function) are defective in a phage-specified recombination system, as shown by two and three-factor crosses carried out in rec + and rec − hosts. 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 erf − mutants after infection of rec − hosts includes failure to grow, to lysogenize, and to sustain phage DNA synthesis beyond about one round of replication. Late phage functions appear to be expressed normally by erf − mutants. Circularization of the infecting circularly permuted, terminally 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 rec − cytoplasm after induction of a lysogen; in this case the DNA 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 suggests that circularization is topologically essential to successful replication of the phage genome.