The structure of recA protein-DNA filaments. 2 recA protein monomers unwind 17 base pairs of DNA by 11.5 degrees/base pair in the presence of adenosine 5'-O-(3-thiotriphosphate).

recA protein binds to duplex DNA in the presence of Mg2+ and adenosine 5'-O-(3-thiotriphosphate) forming a stiff nucleoprotein filament with a distinct axial repeat which contains 17 +/- 1 base pairs and spans 8-9 nm along the fiber (Di Capua, E., Engel, A., Stasiak, A., and Koller, Th. (1982) J. Mol. Biol. 157, 87-103; Dunn, K., Chrysogelos, S., and Griffith, J. (1982) Cell 28, 757-765). Measurement of the protein:DNA ratio in these filaments utilizing double label analysis and isopycnic density banding shows that there are 2 recA monomers for every 17 base pairs. The DNA is also partially unwound in this filament. Utilizing the recA-induced relaxation of naturally supertwisted SV40 DNA, we show that the DNA is unwound by 11.5 +/- 1.5 degrees/base pair which corresponds to 180-200 degrees for each repeat unit along the filament length.