The activity of adenylyltransferase in Rhodospirillum rubrum is only affected by alpha-ketoglutarate and unmodified PII proteins, but not by glutamine, in vitro.

Ammonium assimilation is tightly regulated in nitrogen-fixing bacteria; the target of regulation is primarily the activity of the key enzyme glutamine synthetase that is regulated by reversible covalent modification by AMP groups in reactions catalysed by the bifunctional adenylyltransferase (ATase). The properties and regulation of ATase from Escherichia coli have been studied in great detail. We have investigated the regulation of ATase from Rhodospirillum rubrum, a photosynthetic nitrogen-fixing bacterium. In this diazotroph, nitrogenase is regulated at the metabolic level in addition to the transcriptional regulation operating in all diazotrophic bacteria, which makes understanding the regulatory features of nitrogen assimilation even more interesting. We show that in R. rubrum, in contrast to the E. coli system, ATase is primarily regulated by alpha-ketoglutarate and that glutamine has no effect on neither the adenylylation nor the deadenylylation of glutamine synthetase. Furthermore, the role of the regulatory P(II) proteins is only to stimulate the adenylylation reaction, as there is no effect on the reverse reaction. We propose that in R. rubrum and possibly other diazotrophs alpha-ketoglutarate plays the central role in the regulation of ATase and thus glutamine synthetase activity.