A PP2C-type phosphatase dephosphorylates the PII signaling protein in the cyanobacterium Synechocystis PCC 6803

The family of the PH signal transduction proteins contains the most highly conserved signaling proteins in nature. The cyanobacterial PII-homologue transmits signals of the cellular nitrogen status and carbon status through phosphorylation of a seryl-residue. To identify the enzyme responsible for dephosphorylation of the phosphorylated PH protein in Synechocystis PCC 6803, prospective phosphatase encoding genes were inactivated by targeted insertion of kanamycin resistance cassettes. Disruption of ORF sII1771 generates a mutant unable to dephosphorylate PH under various experimental conditions. On the basis of conserved signature motifs, the sII1771 product (termed PphA) is a member of the protein phosphatase 2C (PP2C) superfamily, which is characterized by Mg2+/Mn2+-dependent catalytic activity. Biochemical analysis of overexpressed and purified PphA confirms its PP2C-type enzymatic properties and demonstrated its reactivity toward the phosphorylated PII protein. Thus, PphA is the first protein phosphatase in Synechocystis PCC 6803 for which the physiological substrate and function is known.