A site-specific, conservative recombination system carried by bacteriophage P1. Mapping the recombinase gene cin and the cross-over sites cix for the inversion of the C segment.

The bacteriophage P1 genome carries an invertible C segment consisting of 3-kb unique sequences flanked by 0.6-kb inverted repeats. With insertion and deletion mutants of P1 derivatives the site-specific recombinase gene cin for C inversion) has been mapped adjacent to the C segment and the cix sites (for C inversion cross-over) have been located at the outside ends of the inverted repeats. Inversion of the C segment functions as a biological switch and controls expression of the gene(s) responsible for phage infectivity carried on the C segment. The cin gene product can promote recombination between a 'quasi- cix ' site on plasmid pBR322 and a cix site on P1 DNA. The junctions formed on the resulting co-integrate can also serve as cix sites. This observation implies a potential evolutionary process to bring genes under the control of a biological switch acting by DNA inversion.