AFM study of RecA-DNA complexes formed on mica surfaces

Kazuo Umemura¹, Shukuko Ikawa², Takehiko Shibata², Mitsuru Ishikawa¹, and Reiko Kuroda^1,3

1) Joint Research Center for Atom Technology (JRCAT), 1-1-1 Higashi, Tsukuba, Ibaraki 305-0046, Japan,
2) The Institute of Physical and Chemical Research (RIKEN), Hirosawa, Wako, Saitama 351-0198, Japan,
3) Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

RecA protein of Escherichia coli optimally promotes DNA strand exchange reactions in the presence of ATP, including the search for homology, homologous pairing and strand transfer. The unique activities are associated with the formation of helical RecA-DNA complexes.
The RecA-DNA complexes have been visualized by electron microscopy in vacuum and by atomic force microscopy (AFM) in air and in aqueous solutions. In all of those studies, the complexes were prepared in a sample tube, and then mounted on grid or mica surfaces for the observation although study of the formation process is a target of the several papers.
In this paper, we studied the formation process of RecA-DNA complexes on mica surfaces. Mixture of DNA molecules and RecA proteins were dropped on a mica surface before incubation, and then incubated on the surface. We found that the formation process is strongly affected by chemical modification of mica surfaces, incubation temperature, and concentration of RecA proteins.