ROLES FOR LAMBDA ORF AND ESCHERICHIA-COLI RECO, RECR AND RECF IN LAMBDA-RECOMBINATION

Bacteriophage lambda lacking its Red recombination functions requires either its own gene product, Orf, or the product of Escherichia coli's recO, recR and recF genes (RecORF) for efficient recombination in rec BC sbcB sbcC mutant cells (the RecF pathway). Phage crosses under cond itions of a partial block to DNA replication have revealed the followi ng: (1) In the presence of Orf, RecF pathway recombination is similar to lambda Red recombination; (2) Orf is necessary for focusing recombi nation toward the right end of the chromosome as X is conventionally d rawn; (3) RecORF-mediated RecF pathway recombination is not focused to ward the right end of the chromosome, which may indicate that RecORF t ravels along the DNA; (4) both Orf-and RecORF-mediated RecF pathway re combination are stimulated by DNA replication; and (5) low level recom bination in the simultaneous absence of Orf and RecORF may occur by a break-copy mechanism that is not initiated by a double strand break. M odels for the roles of Orf and RecO, RecR and RecF in recombination ar e presented.