The Bacillus subtilis recH342 strain, which decreases interspecies recombination without significantly affecting the frequency of transformation with homogamic DNA, carried a point mutation in the putative recX (yfhG) gene, and the mutation was renamed as recX342. We show that RecX (264 residues long), which shares partial identity with the Proteobacterial RecX (<180 residues), is a genuine recombination protein, and its primary function is to modulate the SOS response and to facilitate RecA-mediated recombinational repair and genetic recombination. RecX-YFP formed discrete foci on the nucleoid, which were coincident in time with RecF, in response to DNA damage, and on the poles and/or the nucleoid upon stochastic induction of programmed natural competence. When DNA was damaged, the RecX foci co-localized with RecA threads that persisted for a longer time in the recX context. The absence of RecX severely impaired natural transformation both with plasmid and chromosomal DNA. We show that RecX suppresses the negative effect exerted by RecA during plasmid transformation, prevents RecA mis-sensing of single-stranded DNA tracts, and modulates DNA strand exchange. RecX, by modulating the “length or packing” of a RecA filament, facilitates the initiation of recombination and increases recombination across species.

Initiation of DNA replication must be restricted to occur only once per cell cycle. In most bacteria, DnaA protein binds replication origins and promotes the initiation of DNA replication. We have found that in Bacillus subtilis, DnaA only colocalizes with origin regions at early or late stages of the cell cycle, when the replication machinery is assembling or disassembling, respectively. In contrast, DnaA colocalizes with the DNA replication machinery during most of the cell cycle. Indeed, we present evidence that a primary function of YabA, a negative regulator of replication initiation, is to tether DnaA to the polymerase-clamp protein DnaN. Thus, YabA ensures that once the origin is duplicated, it moves away from the replisome and from DnaA. We propose that DnaA colocalization with origins is specific to the time of initiation, and that replisome/YabA-mediated spatial sequestration of DnaA prevents inappropriate reinitiation of DNA replication.