Effects of different nucleotide cofactors on wild-type (A), K23A (B), and K23A/R33A (C) RecA protein mutants. D-loop formation in the presence of different nucleotide cofactors, as indicated, is shown. (D) K23A/R33A mutant proteins formed a presynaptic nucleoprotein filament with ssDNA in the presence of ATPγS. Negative-staining electron microscopy image shows K23A/R33A proteins with a circular ΦX174 dsDNA in the presence of ATPγS. Scale bar (in black) is 200 nm.