GENETIC-ANALYSIS OF THE BACTERIOPHAGE-LAMBDA ATTL NUCLEOPROTEIN COMPLEX

Site-specific recombination in bacteriophage lambda involves interacti ons among proteins required for integration and excision of DNA molecu les. We have analyzed the elements required to form an in vivo nucleop rotein complex of integrase (Int) and integration host factor (IHF). I nteraction of Int with the core (the site of strand exchange) is stabi lized by the flanking arm region of attL. IHF, in addition to Int, is required for efficient Int-core binding. We used the in vivo attL bind ing assay to characterize several Int variants for their abilities to form stable attL complexes. Substitution of Int active site tyrosine 3 42 by phenylalanine had no effect on the ability of the protein to for m attL complexes. Three other amino acids that are completely conserve d in the integrase family of recombinases (arginine 212, histidine 308 , and arginine 311) were separately substituted by glutamine, leucine, and histidine, respectively. In each case, the mutant protein was alt ered in its ability to form attL complexes while retaining its ability to bind to the lambda arm-type sites. We propose that, in addition to their role in catalysis, this triad of amino acids helps the Int prot ein to interact with the lambda core sites.