The effect of nitrogenase ‘switch-off’ effectors on the concentration of NAD(P)H in Rhodospirillum rubrum G-9 was investigated by fluorescence. A rapid decrease in fluorescence was observed when cells, either N2-grown or nitrogen-starved, were subjected to the effectors, but not when sodium chloride or Tris buffer was added. No effects on the fluorescence were observed in non-nitrogen fixing cultures except when NAD+ was added. The results strongly indicate that the redox state of the pyridine nucleotide pool affects the control of the regulation of nitrogenase activity in R. rubrum.
