Bacteriophage P1 site-specific recombination

The events associated with recombination between loxP, a site in P1 EcoRI fragment 7, and loxB, a site in the bacterial chromosome, are described. When fragment 7 is in a α vector, the product of loxP × loxB recombination is the integration of the λ-P1: 7(loxP) DNA as prophage in the bacterial chromosome and the production of two new hybrid sites, loxR and loxL, flanking the prophage. Lambda phages containing loxL, loxR and loxB sites have been isolated and used to measure recombination frequencies between all four lox sites in paired phage crosses. Two principles emerge from these crosses: (1) loxP and loxL sites are much better substrates for recombination than are loxR and loxB sites. (2) Both partners in a recombination reaction must contain either loxP or loxL sites for recombination to be efficient. These results can explain all of our findings relating to the recombination between α-P1: 7 DNA and the bacterial chromosome. In particular, integration of that DNA into loxB is low because loxP × loxB recombination is inefficient, and excision of the resulting prophage DNA is low because loxL × loxR recombination is inefficient. In contrast, integration of λ-P1: 7 DNA into a chromosome already containing a λ-P1: 7 prophage at loxB is relatively high and is always accompanied by the excision and subsequent loss of the original prophage DNA. We call this process integration by prophage displacement, and we can account for it by postulating that two lox recombination events are involved (loxP × loxR and loxP × loxL), both of which are more efficient than is loxP × loxB recombination. Analysis of the physical structure of the various λlox phages indicates that loxP-containing DNA can integrate into loxB in both possible orientations, and that the loxP × loxB crossover point is located within a 100 base-pair region of P1 BamHI fragment 9, a subfragment of EcoRI fragment 7. The loxB site has been mapped to a region of the Escherichia coli chromosome between tolC (66 min) and dnaG (67 min).