PATHWAYS FOR HOMOLOGOUS RECOMBINATION BETWEEN CHROMOSOMAL DIRECT REPEATS IN SALMONELLA-TYPHIMURIUM

Homologous recombination pathways probably evolved primarily to accomp lish chromosomal repair and the formation and resolution of duplicatio ns by sister-chromosome exchanges. Various DNA lesions initiate these events. Classical recombination assays, involving bacterial sex, focus attention on double-strand ends of DNA. Sexual exchanges, initiated a t these ends, depend on the RecBCD pathway. In the absence of RecBCD f unction, mutation of the sbcB and sbcC genes activates the apparently cryptic RecF pathway. To provide a more general view of recombination, we describe an assay in which endogenous DNA damage initiates recombi nation between chromosomal direct repeats. The repeats flank markers c onferring lactose utilization (Lac(+)) and ampicillin resistance (Ap(R )); recombination generates Lac(-) Ap(S) segregants. In this assay, th e RecF pathway is not cryptic; it plays a major role without sbcBC mut ations. Others have proposed that single-strand gaps are the natural s ubstrate for RecF-dependent recombination. Supporting this view, recom bination stimulated by a double-strand break (DSB) in a chromosomal re peat depended on RecB function, not RecF function. Without RecBCD func tion, sbcBC mutations modified the RecF pathway and allowed it to cata lyze DSB-stimulated recombination. Sexual recombination assays overest imate the importance of RecBCD and DSBs, and underestimate the importa nce of the RecF pathway.