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 mutan t has normal nitrogenase activity under nif-derepressing conditions, i ndicating that ntrBC are not necessary for the expression of the nif g enes in R. rubrum. However, the posttranslational regulation of nitrog enase activity by ADP-ribosylation in response to NH4+ was partially a bolished in this mutant. More surprisingly, the regulation of nitrogen ase activity in response to darkness was also affected, suggesting a p hysiological link between the ntr system and energy signal transductio n in R. rubrum. The expression of glutamine synthetase, as well as its posttranslational regulation, was also altered in this ntrBC mutant.