Characterizing chloroplast sensor kinase

Abstract

In higher plants and green algae, photosynthesis takes place within specialized sub-cellular organelles called chloroplasts. Chloroplasts were once prokaryotes and evolved by endosymbiosis from cyanobacteria. They contain a semi-autonomous genetic system that encodes for core proteins of photosynthetic reaction centres in the energy-transducing membrane known as the chloroplast thylakoid. The photosynthetic apparatus in the thylakoid membrane makes use of excitation energy from sunlight to remove four electrons and protons from two water molecules. The electrons transfer them to the electron acceptor ferredoxin and NADPþ, respectively. In this system, plastoquinone acts as a mobile electron and proton carrier between Photosystem I and Photosystem II in reduction–oxidation or ‘redox’ reactions. A balanced redox state in the chloroplast is important for efficient energy conversion. However, the slightest error could lead to photo-inactivation as well as DNA mutation. Therefore, photosynthetic enzymes that are involved in photosynthesis are tightly regulated. In this study we analyse the mechanism of redox regulation involved in chloroplast gene expression that requires chloroplast sensor kinase (CSK). CSK is a bacterial-like histidine kinase that functions as a two-component system. Such simple but effective signalling transduction is abundant in prokaryotes, but found less widely in eukaryotic cells. CSK is encoded by the nuclear genomes of all higher plants examined, and the CSK proteins are targeted to chloroplasts where they function as a redox sensor. Through the cloning process, the result expressed the full-length CSK and the putative sensor domain (GAF domain) into a pGEX-6P-2 plasmid containing a GST tag. The construction was over-expressed into Escherichia coli cells. From bioinformatics study, it was found that in higher plants CSK is a modified histidine kinase, whereas in diatoms and red algae it is a typical histidine kinase.