The icfG gene cluster of Synechocystis sp strain PCC 6803 encodes an Rsb/Spo-like protein kinase, protein phosphatase, and two phosphoproteins

A set of open reading frames (ORFs) potentially encoding signal transductio n proteins are clustered around icfG, a gene implicated in the regulation o f carbon metabolism, in the genome of Synechocystis sp, strain PCC 6803, sl r1860 is the ORF for icfG, whose predicted product resembles the protein ph osphatases SpoIIE, RsbU, and RsbX from Bacillus subtilis, Bracketing slr186 0/icfG are (i) ORF slr1861, whose predicted product resembles the SpoIIAB, RsbT, and RsbW protein kinases from B. subtilis, and (ii) ORFs slr1856 and slr1859, whose predicted products resemble the respective phosphoprotein su bstrates for the B. subtilis protein kinases: SpoIIAA, RsbS, and RsbV, In o rder to determine whether the protein products encoded by these ORFs posses sed the functional capabilities suggested by sequence comparisons, each was expressed in Escherichia coli as a histidine-tagged fusion protein and ana lyzed for its ability to participate in protein phosphorylation-dephosphory lation processes in vitro. It was observed that ORF slr1861 encoded an ATP- dependent protein kinase capable of phosphorylating Slr1856 and, albeit wit h noticeably lower efficiency, Slr1859, Site-directed mutagenesis suggests that Slr1861 phosphorylated these proteins on Ser-54 and Ser-57, respective ly. Slr1860 exhibited divalent metal ion-dependent protein-serine phosphata se activity. It catalyzed the dephosphorylation of Slr1856, but not Slr1859 , in vitro.