It was discovered that the mutant D112R RecA protein is more resistant to the action of its negative regulator, the RecX protein, than wild type protein both in vitro and in vivo. By means of molecular modeling methods, we showed that amino-acid residue at the position 112 cannot approach the RecX closer than 25-28 Å. Thus, direct contact between the amino acid residue and the RecX is not possible. The RecA D112R protein more actively competes with the SSB protein for the binding sites on single-stranded DNA (ssDNA) and, therefore, differs from wild type RecA by the filamentation dynamics on ssDNA. On the other hand, when replacing ATP to dATP, wild type RecA protein, changing the filamentation dynamics on ssDNA, also become more resistant to the RecX. On the basis of these data, a conclusion was drawn that filamentation dynamics is of substantially greater importance in the resistance of the RecA filament to the RecX than previously discussed protein-protein interactions RecA-RecX. We also propose an improved model of the RecA filament regulation by the RecX protein, according to which the RecA filament elongation along ssDNA is blocked by the RecX protein on the region of ssDNA located beyond the filament.