Activation of recA protein: the salt-induced structural transition.

Purified recA protein is induced by high salt concentrations to hydrolyse ATP even in the absence of DNA. By small angle neutron scattering we show that this salt activation results from a structural transition of the protein filament in the presence of ATP gamma S from the inactive, compact form (a helical polymer of pitch 70 A and cross-sectional radius of gyration Rc 40 A) to the open form (a helical filament of pitch 95 A and Rc 35 A, which are the same structural parameters as in the ATPase active complex with DNA and ATP), without detectable change in the degree of association. We conclude that activation of recA is due to the same structural change whether induced by the binding of DNA or by salt. Indeed, the other enzymatic activity of recA, the proteolytic cleavage of the lexA repressor, is found to be inducible by the same salt concentrations as those of the structural transition.