High [ADP], an indicator of low [ATP], is a strong inhibitor of nitrogenase.
B.
NH 4 + represses the expression of the ~20 nitrogen fixation ( nif ) genes, effectively shutting down the pathway.
C.
Covalent alteration of nitrogenase is also used in some diazotrophs to control nitrogen fixation in response to the availability of NH 4 + in the surroundings.
D.
Transfer of an ADP-ribosyl group from NADH to a specific Arg residue in the nitrogenase reductase shuts down N 2 fixation in Rhodospirillum, for example. This is the same covalent modification that we saw in the case of G protein inhibition by the toxins of cholera and pertussis.