SLP1 is a 17.2-kbp genetic element indigenous to the Streptomyces coelicolor chromosome. During conjugation, SLP1 can undergo excision and subsequent site-specific integration into the chromosomes of recipient cells. We report here the localization, nucleotide sequences, and initial characterization of the genes mediating these recombination events. A region of SLP1 adjacent to the previously identified site of integration, attP, was found to be sufficient to promote site-specific integration of an unrelated Streptomyces plasmid. Nucleotide sequence analysis of a 2.2-kb segment of this region reveals two open reading frames that are adjacent to and transcribed toward the attP site. One of these, the 1,365-bp int gene of SLP1, encodes a predicted 50.6-kDa basic protein having substantial amino acid sequence similarity to a family of site-specific recombinases that includes the Escherichia coli bacteriophage lambda integrase. A linker insertion in the 5' end of the cloned int gene prevents integration, indicating that Int is essential for promoting integration. An open reading frame (orf61) lying immediately 5' to int encodes a predicted 7.1-kDa basic peptide showing limited sequence similarity to the excisionase (xis) genes of other site-specific recombination systems.