The bacteriophage mu 1/6 integrates its DNA into the chromosome of tetracycline producing strains of Streptomyces aureofaciens by a site-specific recombination process. A bioinformatic analysis of the mu 1/6 genome revealed that orf5 encodes a putative integrase, a basic protein of 416 amino acids. The mu 1/6 integrase was found to belong to the integrase family of site-specific tyrosine recombinases. The phage attachment site (attP) was localized downstream of the int gene. The attachment junctions (attL and attR) were determined, allowing identification of the bacterial attachment site (attB). All attachment sites shared a 46-bp common core sequence within which a site-specific recombination occurs. This core sequence comprises the 3' end of a putative tRNA(Thr) gene (anticodon TGT) which is completely restored in attL after integration of the phage into the host genome. An integration vector containing mu 1/6 int-attP region was inserted stably into the S. aureofaciens B96, S. lividans TK24, and S. coelicolor A3. The mu 1/6 integrase was shown to be functional in vivo in heterologous Escherichia coli without any other factors encoded by Streptomyces. In vitro recombination assay using purified mu 1/6 integrase demonstrated its ability to catalyze integrative recombination in the presence of a crude extract of E. coli cells.