EVIDENCE THAT SBCB AND RECF PATHWAY FUNCTIONS CONTRIBUTE TO RECBCD-DEPENDENT TRANSDUCTIONAL RECOMBINATION

A role for the RecF, RecJ, and SbcB proteins in the RecBCD-dependent r ecombination pathway is suggested on the basis of the effect of null r ecF, recJ, and sbcB mutations in Salmonella typhimurium on a ''short-h omology'' P22 transduction assay. The assay requires recombination wit hin short (similar to 3-kb) sequences that flank the selected marker a nd lie at the ends of the transduced fragment. Since these ends are su bject to exonucleolytic degradation, the assay may demand rapid recomb ination by requiring: that the exchange be completed before the essent ial recombining sequences are degraded. In this assay, recF, recJ, and sbcB null mutations, tested individually, cause a small decrease in r ecombinant recovery but all painwise combinations of these mutations c ause a 10- to 30-fold reduction. In a recD mutant recipient, which sho ws increased recombination, these pairwise mutation combinations cause a 100-fold reduction in recombinant recovery. In a standard transduct ion assay (about 20 kb of flanking sequence), recF, recJ, and sbcB mut ations have a very small effect on recombinant frequency. We suggest t hat these three proteins promote a rate-limiting step in the RecBC-dep endent recombination process. The above results were obtained with a l ysogenic recipient strain which represses expression of superinfecting phage genomes and minimizes the contribution of phage recombination f unctions. When a nonlysogenic recipient strain is used, coinfecting ph age genomes express functions that alter the genetic requirements for recombination in the short-homology assay.