RecA protein self-assembly. Multiple discrete aggregation states.

Light scattering, sedimentation and electron microscopy have been used to investigate the aggregation states of highly purified RecA protein in solution. We show that RecA protein will self-assemble into a discrete series of quaternary structures depending upon protein concentration, ionic environment, and nucleotide cofactors. In a stock solution at moderate concentration (10 to 50 microM) RecA protein exists as small particles approximately 4 nm in diameter, larger particles approximately 12 nm in diameter (most probably rings of RecA protein), 10 nm diameter rods varying from 50 to 200 nm in length, and finally as much larger bundles of rods. The addition of monovalent salt shifts the distribution of RecA protein between its various oligomeric states. Increasing protein concentration favors more highly aggregated structures. At a given protein concentration, addition of mM levels of MgCl2 promotes the rapid formation of rods and slow formation of bundles. Under conditions typical of in vitro strand exchange reactions, RecA protein was found to exist as a mixture of rods and 12 nm particles with relatively few monomers.