The integrase (Int) proteins of coliphages HK022 and lambda, are phosphorylated in one or more of their tyrosine residues. In Int of HK022 the phosphorylated residue(s) belong to its core-binding/catalytic domains. Wzc, a protein tyrosine kinase of Escherichia coli, is not required for Int phosphorylation in vivo, however, it can transphosphorylate the conserved Tyr(342) catalytic residue of Int in vitro. Int purified from cells that overexpress Wzc has a reduced activity in vitro. In vivo, the lysogenization of wild type HK022 as well as of lambda is not affected by the overexpression of Wzc. However, the nin5 mutant of lambda, which lacks a protein-tyrosine phosphatase gene, shows a significantly reduced lysogenization. It is suggested that phosphorylation of Int by Wzc down regulates the activity of Int.