Nitrogenase Activity and Membrane Electrogenesis in the Cyanobacterium Plectonema boryanum

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Abstract
The relationship between nitrogenase activity (acetylene reduction) and the transplasmalemma proton electrochemical potential gradient (.DELTA.~.mu.H+) were investigated by using the nonheterocystous filamentous cyanobacterium, P. boryanum. By selectively modifying the chemical (.DELTA.pH) and electrical (.DELTA..psi.) components of .DELTA.~.mu.H+ 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 was 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 energized 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 previous observations using the heterocystous cyanobacterium Anabaena variabilis.