The roles of RecBCD, Ssb and RecA proteins in the formation of heteroduplexes from linear-duplex DNA in vitro.

The formation of heteroduplexes from linear duplex DNA, where one molecule possesses a DNA double-strand break, was assayed by agarose gel electrophoresis. Using unlabeled whole-length linear duplex DNA and 3H-labeled half-length linear duplex DNA (obtained from plasmid pACYC184), the appearance of 3H-labeled DNA that migrated as whole-length linear DNA was taken as evidence for formation of heteroduplex DNA. When the DNA mixtures were incubated with RecA, RecBCD, or Ssb proteins, or any double or triple combination of these proteins under a variety of reaction conditions, no heteroduplex DNA was detected. However, heteroduplex DNA was detected when the DNA mixtures were first incubated briefly with the RecBCD and Ssb proteins under reaction conditions that allow unwinding to proceed, and then the MgCl2 concentration was raised such that renaturation could proceed. The inclusion of the RecBCD and Ssb proteins was sufficient to catalyze the slow formation of heteroduplex DNA, but the presence of RecA protein greatly increased the kinetics. The roles of the RecBCD, Ssb and RecA proteins in heteroduplex formation in vitro are discussed.