URIDYLYLATION OF THE P-II PROTEIN IN THE PHOTOSYNTHETIC BACTERIUM RHODOSPIRILLUM-RUBRUM

The regulatory protein P-II has been studied in great detail in enteri c bacteria; however, its function in photosynthetic bacteria has not b een 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 diazotroph s. In this study, we show that P-II in Rhodospirillum rubrum is modifi ed in response to the N status in the cell and that addition of ammoni um or glutamine leads to demodification. We also provide evidence that P-II is uridylylated. In addition, we show that not only these compou nds but also NAD(+) promotes demodification of P-II, which is of parti cular 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 glut amine synthetase (methionine sulfoximine), whereas treatment with the glutamate synthase inhibitor 6-diazo-5-oxo-norleucine led to total dem odification of P-II without any other addition. The results indicate t hat P-II probably is not directly involved in darkness switch-off of n itrogenase but that a role in ammonium switch-off cannot be excluded.