S-DNA and RecA/RAD51-Mediated Strand Exchange in Vitro.

S-DNA (stretched DNA) is an elongated base-paired DNA conformation under high tension. Because the RecA/Rad51 family DNA recombinases form helical filaments on DNA and mediate the formation of the DNA triplex (D-loop), in which the DNA is stretched, and because the extension of these nucleoprotein filaments is similar to the extension of S-DNA, S-DNA has long been hypothesized as a possible state of DNA that participants in RecA/Rad51-mediated DNA strand exchange in homologous recombination. Such a hypothesis, however, is still lacking direct experimental studies. In this work, we have studied the polymerization and strand exchange on S-DNA mediated by Escherichia coli RecA, human Rad51, and Saccharomyces cerevisiae Rad51 by single-molecule magnetic tweezers. We report that RecA/Rad51 polymerizes faster on S-DNA than on B-DNA with the same buffer conditions. Furthermore, the RecA/Rad51-mediated DNA triplex forms faster from S-DNA than from B-DNA together with the homologous single-stranded DNA. These results provide evidence that S-DNA can interact with RecA and Rad51 and shed light on the possible functions of S-DNA.