Phage X Has an Analog of Escherichia coli rec 0 , recR and recF Genes

The RecF pathway catalyzes generalized recombination in Escherichia coli that is mutant for recBC, sbcB and sbcC. This pathway operating on conjugational recombination requires the recA, recF, recJ, recN, rec0, recQ, recR, ruvA, ruvB and ruvC genes. In contrast, X mutant for its own recombination genes, int, reda and red& requires only the recA and reg genes to recombine efficiently in recBC sbcB sbcC cells. Deletion of an open reading frame in the ninR region of X results in an additional requirement for rec0, recR and recF in order to recombine in recBC sbcB sbcC mutant cells. This function, designated orffor re@-, recBand reef-like function, is largely RecF pathway specific. B ACTERIOPHAGE X growing in Escherichia coli can recombine by any of several different “pathways” catalyzing homologous exchange. These pathways, originally defined for bacterial conjugative recombination, have each been named for a gene(s) essential for recombination in that pathway. Accordingly, the pathways responsible for most recombination in wild-type, recBC sbcB sbcC mutant or recBC sbcA mutant cells have been called the RecBCD, RecF and RecE pathways, respectively (reviewed in CLARK and LOW 1988; MAHAJAN 1988). The “pathway” concept is blurred by observations that plasmids and X differ both from E. coli conjugation and from each other in their genetic requirements for utilization of these pathways (LUISI-DELUCA, LOVETT and KOLODNER 1989; THALER et al. 1989). During bacterial conjugation, recombination by the RecF pathway requires recA, recF, recJ, recN, recO, recQ, recR, ruvA, ruvB and ruvC (HORII and CLARK 1973; LOVETT and CLARK 1984; LLOYD, PRICKSLEY and PRESCOTT 1983; KOLODNER, FISHEL and HOWARD 1985; NAKAYAMA et al. 1984; MAHDI and LLOYD 1989a; LLOYD, BENSON and SHURVINTON 1984; SHARPLES et al. 1990). The recombination of X in recBC sbcB sbcC mutant cells, however, requires recJ but not recF (STAHL et al. 1986; THALER et al. 1989). This paper examines the genetic requirements for other RecF pathway genes when X recombines in recBC sbcB sbcC cells. The observation that X does not require the recF gene for RecF pathway recombination suggested that X encodes an analog of the recF gene product. We looked for analogs of RecF pathway genes in the “nonessential” regions of X. About 40% of X contains accessory genes, which can be deleted with minimal effects on phage development (COURT and OPPENHEIM 1983). The principal regions containing accessory genes are often referred to as the b region, the Genetics 130 7-16 (January, 1992) bio region and the ninR region (see Figure 1). T o determine whether X contains RecF pathway analogs among the accessory genes, we compared the genetic requirements of nondeleted X with X deleted for the b region, the bio region or the ninR region. In contrast to the nondeleted X, we found that X deleted for a single complete open reading frame (ORF) in the ninR region requires recO, recR and recF for recombination in recBC sbcB sbcC mutant cells. The recombinational requirements for X nin5 in recBC sbcB sbcC cells are similar to the requirements for linearized dimer plasmids where recA, recF, recJ, recO, recQand recR (LUISIDELUCA, LOVETT and KOLODNER 1989; N.-W. CHI and R. D. KOLODNER, Harvard Medical school, personal communication) were shown to be necessary. In referring to X recombination in this paper, we define the pathways as follows: (1) X with an int red gam genotype in E. coli mutant for recBC, sbcB and sbcC recombines via the RecF pathway, (2) X with an int red gam genotype in E. coli mutant for recBC and sbcA recombines via the RecE pathway, (3) X with an int red gam genotype in rec+ E. coli recombines via the RecBCD pathway, and (4) X with an int red+ gam+ genotype in E. coli wild type or mutant for recA recombines via the X Red pathway. MATERIALS AND METHODS Phage and bacteria: Bacterial strains are described in Table 1 . P1 lysates were grown as described by MILLER (1972) with transductions performed essentially as in WILLETTS, CLARK and LOW (1969). Samples of each culture used in phage crosses were tested for their recombination, restriction/modification and suppressor genotypes. Mutations in host recombination genes were confirmed by drug resistance and their characteristic UV sensitivity (R. D. KOLODNER, personal communication). recBCD mutants were confirmed by their ability to plate X red gum x+ and X red gam X” with equally large plaques. Mutants of sbcC were confirmed by their ability to plate X gum carrying a palin8 J. A. Sawitzke and F. W. Stahl