Phosphoprotein P II from Cyanobacteria

The signal transduction protein P II from Escherichia coli is modified by uridylylation, whereas its counterpart from the cyanobacterium Synechococcus PCC 7942 is phosphorylated at a seryl residue. To elucidate functional conservations between these proteins, we compared the Synechococcus P II protein with the known properties of the E. coli P II protein. Similar to the E. coli protein, Synechococcus P II binds the metabolites 2‐oxoglutarate and ATP in a mutually dependent manner. The synergism of ligand binding was analyzed in detail. The ATP‐binding site of Synechococcus P II could be labelled with 5′‐ p ‐fluorosulfonylbenzoyladenosine. By heterologous expression of the cyanobacterial glnB gene in E. coli we showed that Synechococcus P II can be modified by the E. coli P II uridylyltransferase. The presence of Synechococcus P II prevents signal transduction of E. coli P II to NtrB, presumably by non‐functional competition. We therefore propose that the primary function of Synechococcus P II is to sense 2‐oxoglutarate, the carbon skeleton required for nitrogen assimilation.