int-h: An int mutation of phage lambda that enhances site-specific recombination

The mutation int-h3 maps in the int gene of coliphage λ and results in the synthesis of an integrase with enhanced activity, which is manifested by an ability to support λ site-specific recombination relatively efficiently under conditions where the wild-type integrase functions inefficiently. The level of site-specific recombination seen in the presence of the int⁺ integrase in himA^? hosts is greatly reduced, as measured by lysogen formation, intramolecular site-specific integration and excision, and excision of a cryptic λ prophage. In contrast, the int-h3 integrase shows relatively high levels of activities under these conditions. Int-h3 is also more active in other host mutants (himB and hip) that reduce λ site-specific recombination. In the absence of the normal attB site, the frequency of lysogen formation (at secondary sites) by λ int⁺ is reduced 200 fold. Although λ int-h3 will integrate preferentially at the attB site if it is present, the mutant phage forms lysogens at a high frequency in attB-deleted hosts. λ int-h3 requires himA function for integration at secondary sites. The fact that the int-h3 integrase uses the same att sites as well as the same host functions as the int⁺ integrase suggests that the mutation results in a quantitative rather than a qualitative change in integrase activity; that is, the int-h3 integrase is more active. The mutant integrase supports site-specific recombination with att sites that carry the att24 mutation. We propose that the int-h3 integrase is endowed with an enhanced ability to recognize att sequences, including some that are not effectively recognized by wild-type integrase.