Integration of bacteriophage Mx8 into the Myxococcus xanthus chromosome causes a structural alteration at the C-terminal region of the IntP protein.

Mx8 is a generalized transducing phage that infects Myxococcus xanthus cells. This phage is lysogenized in M. xanthus cells by the integration of its DNA into the host chromosome through site-specific recombination. Here, we characterize the mechanism of Mx8 integration into the M. xanthus chromosome. The Mx8 attachment site, attP, the M. xanthus chromosome attachment site, attB, and two phage-host junctions, attL and attR, were cloned and sequenced. Sequence alignments of attP, attB, attL, and attR sites revealed a 29-bp segment that is absolutely conserved in all four sequences. The intP gene of Mx8 was found to encode a basic protein that has 533 amino acids and that carries two domains conserved in site-specific recombinases of the integrase family. Surprisingly, the attP site was located within the coding sequence of the intP gene. Hence, the integration of Mx8 into the M. xanthus chromosome results in the conversion of the intP gene to a new gene designated intR. As a result of this conversion, the 112-residue C-terminal sequence of the intP protein is replaced with a 13-residue sequence. A 3-base deletion within the C-terminal region had no effect on Mx8 integration into the chromosome, while a frameshift mutation with the addition of 1 base at the same site blocked integration activity. This result indicates that the C-terminal region is required for the enzymatic function of the intP product.