Cryo electron microscopy of unstained, unfixed RecA-cssDNA complexes

Complexes of RecA protein with φX174 circular single-stranded DNA (cssDNA) with and without ATPγS were rapidly frozen and embedded in a thin layer of vitreous ice. The electron micrographs of these frozen-hydrated complexes clearly show visible helicity. Quantitative image analyses of these micrographs reveal the helical pitch and the axial rise between DNA bases of these complexes. Both of these structural parameters of RecA-cssDNA complexes increase significantly when ATPγS is present. These observations agree qualitatively but not quantitatively with those from negative stained specimens and confirm the general model that the interactions among RecA molecules and between RecA and DNA could change according to the functional states of the RecA-cssDNA complex.