Uridylylation of the P-II protein in the photosynthetic bacterium Rhodospirillum rubrum

The regulatory protein P-II has been studied in great detail in enteric bacteria; however, its function in photosynthetic bacteria has not been clearly established. As a number of these bacteria have been shown to regulate nitrogenase activity by a metabolic control system, it is of special interest to establish the role of P-II in these diazotrophs. In this study, we show that P-II in Rhodospirillum rubrum is modified in response to the N status in the cell and that addition of ammonium or glutamine leads to demodification. We also provide evidence that P-II is uridylylated. In addition, we show that not only these compounds but also NAD(+) promotes demodification of P-II, which is of particular interest as this pyridine nucleotide has been shown to act as a switch-off effector of nitrogenase. Demodification of P-II by ammonium or NAD(+) did not occur in cultures treated with an inhibitor of glutamine synthetase (methionine sulfoximine), whereas treatment with the glutamate synthase inhibitor 6-diazo-5-oxo-norleucine led to total demodification of P-II without any other addition. The results indicate that P-II probably is not directly involved in darkness switch-off of nitrogenase but that a role in ammonium switch-off cannot be excluded.