We have isolated and characterized a novel int mutant of phage λ. This mutant promotes efficient recombination between the phage and bacterial attachment sites, but, unlike wild type, does not promote efficient recombination of any other pair of attachment sites tested in most conditions. IN particular, recombination between two phage or two prophage attachment sites is poor relative to the wild type frequency. We attribute this unusual phenotype to differences in the distribution of int protein binding sites among different attachment sites (Ross and Landy 1982, 1983). We suggest that int protein molecules bound to one of two recombining DNAs interact with empty sites or with bound proteins on the other, and that the mutant protein acts efficiently only if the distribution of protein binding sites within the two attachment sites is that of the attP-attB pair. Similar discrimination among attachment site pairs by wild type int protein may also modulate recombination frequencies.
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