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.