The RecX protein interacts with the RecA protein and modulates its activity in Herbaspirillum seropedicae

DNA repair is crucial to the survival of all organisms. The bacterial RecA protein is a central component in the SOS response and in recombinational and SOS DNA repairs. The RecX protein has been characterized as a negative modulator of RecA activity in many bacteria. The recA and recX genes of Herbaspirillum seropedicae constitute a single operon, and evidence suggests that RecX participates in SOS repair. In the present study, we show that the H. seropedicae RecX protein (RecX_Hs) can interact with the H. seropedicae RecA protein (RecA_Hs) and that RecA_Hs possesses ATP binding, ATP hydrolyzing and DNA strand exchange activities. RecX_Hs inhibited 90% of the RecA_Hs DNA strand exchange activity even when present in a 50-fold lower molar concentration than RecA_Hs. RecA_Hs ATP binding was not affected by the addition of RecX, but the ATPase activity was reduced. When RecX_Hs was present before the formation of RecA filaments (RecA-ssDNA), inhibition of ATPase activity was substantially reduced and excess ssDNA also partially suppressed this inhibition. The results suggest that the RecX_Hs protein negatively modulates the RecA_Hs activities by protein-protein interactions and also by DNA-protein interactions