THE ROLE OF NAD(-OFF IN RHODOSPIRILLUM-RUBRUM() AS A SIGNAL DURING NITROGENASE SWITCH)

The role of NAD(+) in the metabolic regulation of nitrogenase, the 'sw itch-off' effect, in Rhodospirillum rubrum has been studied. We now sh ow that the decrease in nitrogenase activity upon addition of NAD(+) t o R. rubrum is due to modification of dinitrogenase reductase. There w as no effect when NAD(+) was added to a mutant of R. rubrum devoid 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 alpha-ketoglutarate fu nction as 'switch-off' effecters. On the other hand beta-hydroxybutyra te has the opposite effect by shortening the 'switch-off' period. Furt hermore, 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(+) a re involved in initiating 'switch-off'.