BLOCKED RECA PROTEIN-MEDIATED DNA STRAND EXCHANGE-REACTIONS ARE REVERSED BY THE RUVA AND RUVB PROTEINS

RecA protein is unable to complete a DNA strand exchange reaction betw een a circular single-stranded DNA and a linear duplex DNA substrate w ith heterologous sequences of 375 base pairs at the distal end. Instea d, it generates a branched intermediate in which strand exchange has p roceeded up to the homology/heterology junction, Addition of the RuvA and RuvB proteins to these stalled intermediates leads to the rapid co nversion of intermediates back to the original substrates, The reversa l reaction is initiated at the branch, and the hybrid DNA is unwound i n the direction opposite to that of the RecA reaction that created it, Under optimal conditions the rate of the reaction exhibits only a mod est dependence on the length of hybrid DNA that must be unwound. Produ cts of the reversal reaction are detected within minutes after additio n of RuvAB, and appear with an apparent first order progress curve exh ibiting a t(1/2) in the range of 6-12 min under optimal conditions. Fe w molecules that have undergone only partial reversal are detected, Th is suggests that the assembly or activation of RuvAB on the branched s ubstrate is rate-limiting, while any migration of RuvAB on the DNA to effect unwinding of the hybrid DNA (and reformation of substrate DNA) is very fast. The results are discussed in the context of the role of RuvA and RuvB proteins in recombinational DNA repair. We suggest that one function of the RuvAB proteins is to act as an antirecombinase, to eliminate intragenomic crossovers between homologous segments of the bacterial chromosome that might otherwise lead to deleterious inversio ns or deletions.