Bacteriophage X can recombine in recBC sbcB sbcC mutant cells by using its own gene orj; the Escherichia coli recO, recR, and recF genes, or both. Expression of an orf-containing plasmid complements the recombination defects of orf mutant phage. However, this clone does not complement a recO mutation for conjugational recombination or recO, recR, or recF mutations for repair of UV-induced DNA damage. A plasmid clone of orf produces a protein with an apparent molecular mass of 15 kDa.Recombination in an Escherichia coli recBC sbcB sbcC mutant background is said to be catalyzed by the RecF pathway (see references 7 and 36 for reviews). The genetic requirements for operation of the RecF pathway vary with different assays, however. For example, recombination after conjugational DNA transfer into recBC sbcB sbcC mutant cells requires the recA, recF, recG, rec, recN, recO, recQ, recR, ruvA, ruvB, and ruvC genes (14,15,23,25,27,30,37,42,46), whereas recombination within linearized dimer plasmids after transformation requires only a subset of those genes, namely, recA, recF, rec, recO, recQ, and recR (6,34). In contrast, bacteriophage X, mutant for its own recombination system, Red, requires only the recA and rec genes to recombine by the RecF pathway (45).The multiple pathway concept of recombination was formulated (in part) to explain why mutations in some recombination genes conspicuously affect recombination in some genetic backgrounds but have little or no effect in others. With the exception of recA, all of the RecF pathway genes mentioned above have little effect on conjugational recombination in wild-type (recBC' sbcB+ sbcC+) cells. However, the RecF pathway genes recA, recF, rec, recO, and recR are required for plasmid recombination in wild-type cells (15,37