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Archaeal RadA or Rad51 recombinases are close homologues of eukaryal Rad51 and DMC1. These and bacterial RecA orthologues play a key role in DNA repair by forming helical nucleoprotein filaments in which a hallmark strand exchange reaction between homologous DNA substrates occurs. Recent studies have discovered the stimulatory role by calcium on human and yeast recombinases. Here we report that the strand exchange activity but not the ATPase activity of an archaeal RadA/Rad51 recombinase from Methanococcus voltae (MvRadA) is also subject to calcium stimulation. Crystallized MvRadA filaments in the presence of CaCl2 resemble that of the recently reported ATPase active form in the presence of an activating dose of KCl. At the ATPase center, one Ca2+ ion takes the place of two K+ ions in the K+-bound form. The terminal phosphate of the non-hydrolyzable ATP analogue is in a staggered conformation in the Ca2++bound form. In comparison, an eclipsed conformation was seen in the K+-bound form. Despite the changes in the ATPase center, both forms harbor largely ordered L2 regions in essentially identical conformations. These data suggest a unified stimulation mechanism by potassium and calcium because of the existence of a conserved ATPase center promiscuous in binding cations. [ABSTRACT FROM AUTHOR]