Bacillus subtilis RecA interacts with and loads RadA/Sms to unwind recombination intermediates during natural chromosomal transformation.

During natural transformation Bacillus subtilis RecA, polymerized onto the incoming single-stranded (ss) DNA, catalyses DNA strand invasion resulting in a displacement loop (D-loop) intermediate. A null radA mutation impairs chromosomal transformation, and RadA/Sms unwinds forked DNA in the 5'→3' direction. We show that in the absence of RadA/Sms competent cells require the RecG translocase for natural chromosomal transformation. RadA/Sms tetracysteine motif (C13A and C13R) variants, which fail to interact with RecA, are also deficient in plasmid transformation, but this defect is suppressed by inactivating recA. The RadA/Sms C13A and C13R variants bind ssDNA, and this interaction stimulates their ATPase activity. Wild-type (wt) RadA/Sms interacts with and inhibits the ATPase activity of RecA, but RadA/Sms C13A fails to do it. RadA/Sms and its variants, C13A and C13R, bound to the 5'-tail of a DNA substrate, unwind DNA in the 5'→3' direction. RecA interacts with and loads wt RadA/Sms to promote unwinding of a non-cognate 3'-tailed or 5'-fork DNA substrate, but RadA/Sms C13A or C13R fail to do it. We propose that wt RadA/Sms interaction with RecA is crucial to recruit the former onto D-loop DNA, and both proteins in concert catalyse D-loop extension to favour integration of ssDNA during chromosomal transformation.