Recombination in bacteriophage P2: recA dependent enhancement by ultraviolet irradiation and by transfection with mixed DNA dimers.

Bacteriophage P2 is known for its exceptionally low rate of spontaneous (non-integrative) recombination, which however may be stimulated by ultraviolet irradiation of the phage. We show here that ligated dimers, made in vitro from mixtures of DNAs of two P2 mutants, upon transfection of lysozyme-spheroplasts give origin to recombinants at high frequency. While spontaneous P2 recombination occurs independently of the main recombination pathway of the bacteria, P2 recombinant formation following either ultraviolet irradiation or transfection with DNA dimers requires at least some element of such a pathway, since it is absent or greatly reduced in recA- bacteria or spheroplasts. It would seen that, in the course of its lytic development, P2 deploys a mechanism that inhibits the main recombination pathway of the host cell, or assumes DNA configurations refractory to it.