Phosphorylation of proteins in the light-dependent signalling pathway of a filamentous cyanobacterium

The genome of the filamentous cyanobacterium Calothrix sp. PCC7601 contains two genes, cphA and cphB, encoding proteins with similarity to plant phytochromes and bacterial histidine kinases. In vitro, CphA and CphB readily attach a tetrapyrrole chromophore to develop spectrally active holoproteins that are photointerconvertible between a red light-absorbing and a far-red light-absorbing form. Together with the putative response regulators, RcpA and RcpB, the putative histidine kinases, CphA and CphB, are suggested to constitute two two-component systems of light-dependent signal transduction. In this report, we demonstrate the kinase activity of both CphA and CphB. In vitro experiments carried out on the purified proteins show that CphA and CphB are autophosphorylated in the presence of ATP and that phospho-CphA is capable of efficient phosphotransfer to RcpA as is phospho-CphB towards RcpB. The autophosphorylation and the phosphorelay are dependent on light. Both activities are reduced under red light vs. far-red light irradiation. No phosphoryl transfer occurred between phospho-CphA and RcpB or between phospho-CphB and RcpA. The response regulators RcpA and RcpB can receive a phosphoryl moiety also from the small phospho-donor acetyl phosphate. The stability of the phosphorylated regulators is not affected by CphA and CphB or light.