Stable complexes were formed between the recA protein of Escherichia coli and duplex DNA in the presence of adenosine 5′-γ-thiotriphosphate. From the known number of base-pairs of the plasmid used, from the appearance of the complexes in the electron microscope after platinum shadowing and negative staining and from mass determinations in situ by scanning transmission electron microscopy, we deduce a structure in which 18.6 base-pairs and 6.4 recA monomers contribute to one turn of a right-handed helix with a pitch of 9.6 nm and a width of 11 nm. The results suggest an intercalative mode of binding, which partially unwinds the DNA.
ATP[S], adenosine 5′-γ-thiotriphosphate; STEM, scanning transmission electron microscope(y); FWHM, full-width half-maximum