Nitrogenase activity and membrane electrogenesis in the cyanobacterium Plectonema boryanum

The relationships between nitrogenase activity (acetylene reduction) and the transplasmalemma proton electrochemical potential gradient (delta muH+) have been investigated using the non-heterocystous filamentous cyanobacterium Plectonema boryanum. By selectively modifying the chemical (delta pH) and electrical (delta psi) components of delta muH+ under conditions in which the size of the ATP pool remained unaffected, nitrogenase activity was found to be dependent on, or regulated by delta psi. When the ATP pool decreased, concomitant with a decreased internal pH (pHi) the requirement for delta psi was modified. The observed reduction in the intracellular ATP pool and the decreased ATP:ADP ratio also correlated with an inhibition of nitrogenase activity. The data are consistent with a model in vivo in which reductant supply to nitrogenase is regulated by, or dependent on an energised plasmalemma and where there is a fine balance between the supply of reductant and ATP for nitrogenase activity. The correlation observed between delta psi and nitrogenase activity extends our previous observations using the heterocystous cyanobacterium Anabaena variabilis.