EVIDENCE FOR ELONGATION OF THE HELICAL PITCH OF THE RECA FILAMENT UPON ATP AND ADP BINDING USING SMALL-ANGLE NEUTRON-SCATTERING

Structural changes of the RecA filament upon binding of cofactors have been investigated by small-angle neutron scattering. Both ATP and ADP increased the helical pitch of the RecA homopolymer, which is observe d to be 7 nm in the absence of any cofactor. The binding of ATP altere d the pitch to 9 nm, whereas the binding of ADP only produced a pitch of 8.2 nm. The pitch determined for the RecA complex with the ATP anal og adenosine 5'-[gamma-thio]triphosphate was similar to that found wit h ATP. Thus, at least three, somewhat different, RecA helical filament ous structures may form in solution. The binding of DNA to RecA did no t alter the pitch significantly, indicating that the cofactor binding is the determining factor for the size of the helical pitch of the Rec A filament. We also found that elongation of the helical pitch is a ne cessary, but not a sufficient condition, for the coprotease activity o f RecA. The presence of acetate or glutamate ions is also required. Th e pitch of the ADP . RecA filament is in agreement with that found in the crystal structure. This correlation indicates that this structure corresponds to that of the ADP . RecA filament in solution, although t his is not the species active in recombination.