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Abstract
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.