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Streptomyces phage ~C3l produces a site-specific integrase by whiCh it is able to integrate its DNA into the host chromosome. ~C3l integrase belongs to the serine recombinase family and utilizes small (less than 50 base pair) substrates for recombination. ~C3l integrase (in vitro) is a unidirectional enzyme which can recombine attP and attB sites to form hybrid product sites; attL and attR.The interaction ofintegrase with its attB site was studied in detail wherein the attB site was subjected to mutagenesis and the activities of the mutants assayed in functional assays. Mutational analysis of the attB site revealed the presence of bases at positions 2, 15 and 16 which severely affected the recombination efficiency. Mutations at bases 15 and 16 were however found to be suppressible at high NaCI concentrations.