PROPERTIES OF THE PERIBACTEROID MEMBRANE ATPASE OF PEA ROOT-NODULES AND ITS EFFECT ON THE NITROGENASE ACTIVITY

Peribacteroid membrane vesicles from pea (Pisum sativum) root nodules were isolated from membrane-enclosed bacteroids by an osmotic shock. T he ATPase activity associated with this membrane preparation was chara cterized, and its electrogenic properties were determined. The pH grad ient was measured as a change of the fluorescence intensity of 9-amino -6-chloro-2-methoxyacridine and the membrane potential as a shift of a bsorbance of bis-(3-propyl-5-oxoisoxazol-4-yl)pentamethine oxonol. It was demonstrated that the ATPase generates a pH gradient as well as a membrane potential across the peribacteroid membrane. The reversibilit y of the ATPase was demonstrated by a light-dependent ATP synthesis by peribacteroid membrane vesicles fused with bacteriorhodopsin-phosphol ipid vesicles. The light-driven AIP synthesis by the peribacteroid mem brane ATPase was completely inhibited by a proton-conducting ionophore . The proton-pumping activity of the peribacteroid membrane ATPase cou ld also be demonstrated with peribacteroid membrane-enclosed bacteroid s, and effects on nitrogenase activity were established. At pH values below 7.5, an active peribacteroid membrane ATPase inhibited the nitro genase activity of peribacteroid membrane-enclosed bacteroids. At pH v alues above 8, at which whole cell nitrogenase activity was inhibited, the proton-pumping activity of the peribacteroid membrane ATPase coul d partially reverse the pH inhibition. Vanadate, an inhibitor of plasm a membrane and peribacteroid membrane ATPases, stimulated nodular nitr ogenase activity. It will be proposed that the proton-pumping activity of the peribacteroid membrane ATPase in situ is a possible regulator of nodular nitrogenase activity.