Figure 1.

Versatility of the site-specific recombination system of phage lambda and recombination scenarios between att sites located on the E. coli chromosome. (A) Integrative and excisive recombination promoted by ‘Int' and ‘Int+Xis'. In the presence of Int, recombination between attB and attP sites generates attL and attR sites. In the absence of Xis, the inverted configuration is blocked and restoration of the initial state will be possible only in the presence of Int and Xis. (B) attP and attB cassettes allowing phenotypic detection of recombined fragments. A 27-bp fragment corresponding to λ attB has been inserted in-frame in the lacZ coding region. Integrative recombination disrupts lacZ integrity. A reciprocal transaction between attL and attR cassettes restores lacZ integrity. (C) Recombination between directly repeated att sites. Recombination between directly repeated attL and attR sites located on the same chromosome results in a deletion of the intervening fragment A (1, excisive deletion). Recombination between attL and attR sites located on different chromatids provokes duplication of the intervening fragment A on one of the two chromatids and deletion of the same fragment on the other chromatid (2). Identity of the att site between the duplicated fragments (attB or attP, attB in the example shown) varies with the position of attL relative to that of attR. (D) Recombination between inverted attL and attR sites located on the same chromosome results in inversion of the intervening fragment A (1, excisive inversion). Recombination between attL and attR sites located on different chromatids provokes the formation of a palindromic chromosome dimer presumably lethal (2) (Nash, 1996).