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 tr ansmits signals of the cellular nitrogen status and carbon status through p hosphorylation of a seryl-residue. To identify the enzyme responsible for d ephosphorylation of the phosphorylated PH protein in Synechocystis PCC 6803 , prospective phosphatase encoding genes were inactivated by targeted inser tion of kanamycin resistance cassettes. Disruption of ORF sII1771 generates a mutant unable to dephosphorylate PH under various experimental condition s. On the basis of conserved signature motifs, the sII1771 product (termed PphA) is a member of the protein phosphatase 2C (PP2C) superfamily, which i s characterized by Mg2+/Mn2+-dependent catalytic activity. Biochemical anal ysis of overexpressed and purified PphA confirms its PP2C-type enzymatic pr operties and demonstrated its reactivity toward the phosphorylated PII prot ein. Thus, PphA is the first protein phosphatase in Synechocystis PCC 6803 for which the physiological substrate and function is known.