SALMONELLA RECD MUTATIONS INCREASE RECOMBINATION IN A SHORT SEQUENCE TRANSDUCTION ASSAY

We have identified recD mutants of Salmonella typhimurium by their abi lity to support growth of phage P22 abe (anti-RecBCD) mutants, whose g rowth is prevented by normal host RecBCD function. As in Escherichia c oli, the recD gene of S. typhimurium lies between the recB and argA ge nes at min 61 of the genetic map. Plasmids carrying the Salmonella rec BCD(+) genes restore ATP-dependent exonuclease V activity to an E. col i recBCD deletion mutant. The new Salmonella recD mutations (placed on this plasmid) eliminate the exonuclease activity and enable the plasm id-bearing E. coli deletion mutant to support growth of phage T4 gene 2 mutants. The Salmonella recD mutations caused a 3- to 61-fold increa se in the ability of a recipient strain to inherit (by transduction) a large inserted element (MudA prophage; 38 kb). In this cross, recombi nation events must occur in the short (3-kb) sequences that flank the element in the 44-kb transduced fragment. The effect of the recD mutat ion depends on the nature of the flanking sequences and is likely to b e greatest when those sequences lack a Chi site. The recD mutation app ears to minimize fragment degradation and/or cause RecBC-dependent rec ombination events to occur closer to the ends of the transduced fragme nt. The effect of a recipient recD mutation was eliminated if the dono r P22 phage expressed its Abe (anti-RecBC) function. We hypothesize th at in standard (high multiplicity of infection) P22-mediated transduct ion crosses, recombination is stimulated both by Chi sequences (when p resent in the transduced fragment) and by the phage-encoded Abe protei n which inhibits the host RecBCD exonuclease.