The role of NAD⁺ as a signal during nitrogenase switch-off in Rhodospirillum rubrum

The role of NAD⁺ in the metabolic regulation of nitrogenase, the ‘switch-off’ effect, in Rhodospirillum rubrum has been studied. We now show that the decrease in nitrogenase activity upon addition of NAD⁺ to R. rubrum is due to modification of dinitrogenase reductase. There was no effect when NAD⁺ was added to a mutant of R. rubrumdevoid of dinitrogenase reductase ADP-ribosyltransferase, indicating that NAD⁺ ‘switch-off’ is an effect of the same regulatory system as ammonium ‘switch-off’. We also show that oxaloacetate and α-ketoglutarate function as ‘switch-off’ effectors. On the other hand β-hydroxybutyrate has the opposite effect by shortening the ‘switch-off’ period. Furthermore, by using an inhibitor of glutamate synthase the role of this enzyme in ‘switch-off’ was investigated. The results are discussed in relation to our proposal that changes in the concentration of NAD⁺ are involved in initiating ‘switch-off’.