Core-binding specificity of bacteriophage integrases

The site-specific recombination systems of bacteriophages λ and HK022 share the same mechanism and their integrase proteins show strong homology. Nevertheless the integrase protein of each phage can only catalyze recombination between its own att sites. Previous work has shown that the specificity determinants in the att sites are located within the sequences that bind the integrase to the core of att. DNA fragments that carry attL and attR sites of each phage were challenged with each of the two integrases and the DNA-protein complexes were examined by the gel-retardation technique. The results show that each integrase can form higher-order DNA-protein complexes only with its cognate att sites, suggesting that differences in the mode of binding to the core are responsible for the specificity difference between the two integrases.