Site-specific Recombination and Its Role in the Life Cycle of Bacteriophage P1

A variety of bacteriophages, including λ, P2, P22, and Mu, encode site-specific recombination systems. The need for these systems is clear because the normal prophage DNA of these viruses is integrated into the bacterial chromosome. Integration occurs at specific sites within the phage DNA, and in the cases of λ, P2, and P22 also within the bacterial chromosome. In contrast, the prophage DNA of bacteriophage P1 is an autonomous plasmid. Consequently, the need for and the role of a P1 site-specific recombination system are not at all obvious. Despite this, the following observations suggest that P1 encodes such a system. First, the viral DNA of P1, like that of phages P22 and T4, is terminally redundant and cyclically permuted (Ikeda and Tomizawa 1969). Thus, one could expect that the genetic map of P1, like those of P22 and T4, would be circular; in fact, it is linear (Scott 1968; Walker...