Identification of site-specific recombination genes int and xis of the Rhizobium temperate phage 16-3

Phage 16-3 is a temperate phage of Rhizobium meliloti 41 which integrates i ts genome with high efficiency into the host chromosome by site-specific re combination through DNA sequences of attB and attP. Here we report the iden tification of two phage-encoded genes required far recombinations at these sites: int (phage integration) and xis (prophage excision). We concluded th at Int protein of phage 16-3 belongs to the integrase family of tyrosine re combinases. Despite similarities to the cognate systems of the lambdoid pha ges, the 16-3 int xis aff system is not active in Escherichia coil, probabl y due to requirements for host factors that differ in Rhizobium meliloti an d E. coli. The application of the 16-3 site-specific recombination system i n biotechnology is discussed.