Visualization of RecA-DNA Complexes Involved in Consecutive Stages of An In Vitro Strand Exchange Reaction

RecA protein, which is essential for general genetic recombination in Escherichia coli, in vitro pairs single-stranded DNA (ssDNA) with homologous double-stranded DNA (dsDNA) and promotes strand exchange (for review and other recombination reactions mediated by RecA, see Radding 1982). This reaction has been shown to require ATP hydrolysis and to be stimulated by the single-strand-binding protein (SSB) (Cox and Lehman 1981). In the presence of the non-hydrolyzable analog of ATP adenosine-5′-O-(3-thiotriphosphate) (ATPγS), dsDNA is found covered with RecA in stable complexes (West et al. 1980; Stasiak et al. 1981; Dunn et al. 1982; Flory and Radding 1982), at a stoichiometry of 1 RecA/3 bp (DiCapua et al. 1982; Dombrowski et al., 1983). These filamentous RecA-DNA complexes are helical, and the dsDNA in the complexes is unwound from 10.5 bp/turn (B-DNA) into 18.5 bp/turn, following the protein helix (Stasiak and DiCapua 1982).