Lysogenization by a clts variant of coliphage P1, Plcl.100, markedly increased the frequency of reversion of a gaiT::ISI mutation. The formation of Gal' colonies presumably occurs by microhomologous recombination between the 9-base-pair repeats in gafT (CGCCGCTAC) Several temperate coliphages are known to express recombination functions. The int and xis genes of lambda, for example, specify site-specific recombination enzymes, while the reda and redb gene products, X exonuclease and beta protein, respectively, promote homologous recombination. The four genes are clustered in the lambda genome, and their roles in lambda physiology are known; the first two are required for prophage integration and excision, whereas the products of the red genes convert lambda monomers into packageable multimers.Prophage P1 expresses a site-specific recombination function, Cre, which improves P1 plasmid maintenance by cutting and rejoining P1 chromosomes at the P1 lox sites (5,16,18). RecA is responsible for most P1 circularization, since Cre can circularize only those P1 molecules, about 20% of the total, which bear redundant lox sites. In recA mutants, however, Cre is essential for P1 lysogenization and lytic growth.The lytic development of P1 prophage is blocked by the bacteriophage-encoded cI gene product. Lysogens carrying Plcl. 100, a clts mutant, are stable at 32°C but are induced at temperatures above 39°C (13). Several years ago, we observed that Plcl.100 lysogens displayed a phenotype related to recombination. Escherichia coli bearing the IS] gal operon insertion, galTN102 (6), revert to Gal' at elevated frequencies when lysogenic for Plclts. We assume that this reversion results from recombination between the repeated 9-base-pair (bp) galT target sequences which flank the IS] transposon, a process referred to as precise excision. Independently, Windle and Hays (19) noted that recombination between two close but nonoverlapping deletions in lacZ was likewise increased by Plcl.100 prophage. Plcl.100 did not appear to stimulate recombination within large regions of DNA homology (19). These observations suggested that P1 carries a recombination enhancement function (re]) which