P1 site-specific recombination: nucleotide sequence of the recombining sites.

Site-specific recombination between molecules of bacteriophage P1 DNA occurs at sites called loxP and requires the action of a protein that is the product of the P1 cre gene. Although recombination between two loxP sites is very efficient, recombination between loxP and a unique site in the bacterial chromosome (loxB) is inefficient and generates two hybrid lox sites called loxR and loxL. We present here the nucleotide sequences of all four lox sites. Analysis of these sequences indicates that (i) a region of extensive homology is not present at the loxP X loxB crossover point, in contrast to the 15-base pair common-core sequence in the bacteriophage lambda att sites, and (ii) the sites contain a region of dyad symmetry with 8- to 13-base pair inverted repeats separated by an 8- to 9-base pair sequence. The loxP X loxB crossover point falls in the sequence that separates the inverted repeats, and deletions that remove either the left or the right inverted repeat of loxP inactivate the site. These two observations are consistent with the conclusion that the region of dyad symmetry is important in los recombination. We have shown further that the loxP X loxP crossover point occurs in a 63-base pair sequence containing the loxP X loxB crossover point, suggesting that, despite the great difference in efficiencies of the two reactions, the crossover points may occur at the same place in both. Explanations for the different recombination properties of the various lox sites are discussed.