Phosphoprotein P-II from cyanobacteria - Analysis of functional conservation with the P-II signal-transduction protein from Escherichia coli

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.