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 recBC sbcB sbcC mutant cells (the RecF pathway). Phage crosses under conditions of a partial block to DNA replication have revealed the following: (1) In the presence of Orf, RecF pathway recombination is similar to lambda Red recombination; (2) Orf is necessary for focusing recombination toward the right end of the chromosome as lambda is conventionally drawn; (3) RecORF-mediated RecF pathway recombination is not focused toward the right end of the chromosome, which may indicate that RecORF travels along the DNA; (4) both Orf- and RecORF-mediated RecF pathway recombination are stimulated by DNA replication; and (5) low level recombination in the simultaneous absence of Orf and RecORF may occur by a break-copy mechanism that is not initiated by a double strand break. Models for the roles of Orf and RecO, RecR and RecF in recombination are presented.