The P_II Protein in Synechococcus PCC 7942 Senses and Signals 2-Oxoglutarate Under ATP-Replete Conditions

Abstract

The glnBgene product, termed P_II protein, is widely distributed among bacteria and functions as a signal transduction protein in the central regulation of nitrogen metabolism (reviewed in 9). In proteobacteria, P_II is modified by uridylylation at a conserved tyrosyl residue (Tyr 51); under nitrogen replete conditions, P_II is present in its unmodified state whereas P_II-UMP signals nitrogen-deficiency. Recently the 3-D structure of P_II from Escherichia colihas been resolved (1), showing that the site of modification is located at the apex of a large solvent-protruding loop, termed T-loop. In contrast to proteobacteria, P_II in cyanobacteria is not modified by uridylylation but is phosphorylated at a seryl residue (Ser 49) separated only by one amino acid from the conserved tyrosyl residue (2,4). This indicates that phosphorylation also occurrs at the solvent-exposed T-loop. In SynechococcusPCC 7942, the trimeric P_II protein was shown to be involved in the coordination of carbon and nitrogen assimilation, in particular mediating the dependence on CO₂ fixation for nitrate utilization (3). In vivo analyses revealed that the phosphorylation state of P_II responds to the status of nitrogen and carbon assimilation (2,3). In the presence of ammonium, P_IIis predominantly present in its unmodified form. In nitrate grown cells, the extent of P_II phosphorylation depends on the CO₂ supply to the cells: Under CO₂-limiting conditions, only a low degree of P_II phosphorylation is observed whereas under CO₂ sufficiency, P_II is efficiently phosphorylated. The highest level of phosphorylation is found in nitrogen-starved cells. Studying the P_II modification system, therefore, offers the possibility to investigate a mechanism used by cyanobacteria to sense the environmental changes in the nitrogen and carbon supply.