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

Abstract

The glnBgene product, termed PII 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, PII is modified by uridylylation at a conserved tyrosyl residue (Tyr 51); under nitrogen replete conditions, PII is present in its unmodified state whereas PII-UMP signals nitrogen-deficiency. Recently the 3-D structure of PII 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, PII 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 PII 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 PII responds to the status of nitrogen and carbon assimilation (2,3). In the presence of ammonium, PIIis predominantly present in its unmodified form. In nitrate grown cells, the extent of PII phosphorylation depends on the CO2 supply to the cells: Under CO2-limiting conditions, only a low degree of PII phosphorylation is observed whereas under CO2 sufficiency, PII is efficiently phosphorylated. The highest level of phosphorylation is found in nitrogen-starved cells. Studying the PII modification system, therefore, offers the possibility to investigate a mechanism used by cyanobacteria to sense the environmental changes in the nitrogen and carbon supply.