Effect of an ntrBC mutation on the posttranslational regulation of nitrogenase activity in Rhodospirillum rubrum.

Homologs of ntrB and ntrC genes from Rhodospirillum rubrum were cloned and sequenced. A mutant lacking ntrBC was constructed, and this mutant has normal nitrogenase activity under nif-derepressing conditions, indicating that ntrBC are not necessary for the expression of the nif genes in R. rubrum. However, the post-translational regulation of nitrogenase activity by ADP-ribosylation in response to NH4+ was partially abolished in this mutant. More surprisingly, the regulation of nitrogenase activity in response to darkness was also affected, suggesting a physiological link between the ntr system and energy signal transduction in R. rubrum. The expression of glutamine synthetase, as well as its posttranslational regulation, was also altered in this ntrBC mutant.