Member of possibly 2 two-component regulatory system(s) Hik2/Rre1 and Hik2/RppA. Transduces PQ (plastoquinone) redox signals to photosystem gene expression machinery during the adjustment of photosystem stoichiometry. Reduced PQ suppresses its autophosphorylation activity (i.e. kinase activity is higher under oxidizing conditions) (Probable). Member of two-component regulatory system Hik2/Rre1, controls expression of sigB (sll0306), sll0528, slr1119, slr0852 and ssr3188 in response to hyperosmotic stress (Probable). Activity responds to high salt (with a linear response as concentrations rise to 0.5 M NaCl); detects Cl(-) levels (PubMed:24283389). Autophosphorylates and transfers phosphate to Rre1 (PubMed:25714549, PubMed:26904089). May transfer phosphate to RppA in a possible Hik2/RppA two-component system (Probable). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. The 3Fe-4S cluster is redox-responsive, binds 1 cluster per monomer. It is coordinated by Cys-19 and other non-Cys residues. Autophosphorylation is inhibited by Na(+) but not by Cl(-) (PubMed:26904089). Reducing agents dithionite, duroquinol and decyl-plastoquinone, but not NADPH or ferredoxin inhibit autophosphorylation. Oxidation of the Fe-S cluster (with potassium ferricyanide) induces a conformational change that is conducive to its autophosphorylation activity (PubMed:31925322). E(0) is -15 mV. Exists as monomers, tetramers, hexamers and other higher-order oligomers; all are able to autophosphorylate. Upon treatment with 0.5 M NaCl only tetramers are seen, which are probably inactive (PubMed:29290041). Interacts with both RppA and Rre1 (PubMed:26904089). The N-terminal GAF domain activated by Cl(-). Autophosphorylates, probably on His-185. Essential, it cannot be deleted. Belongs to the chloroplast sensor kinase protein family.