Member of 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). As part of a two-component regulatory system with Rre1, controls expression of sigB and several other genes in response to hyperosmotic stress. May transfer phosphate to RppA in a possible Hik2/RppA two-component system. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. The 3Fe-4S cluster is redox-responsive, binds 1 cluster per monomer. Hexamers; upon treatment with 0.5 M NaCl only tetramers are seen. The tetramers are probably inactive. Oligomerization seems to be mediated by the histidine kinase domain (residues 142-386) which forms tetramers and hexamers. The DHp subdomain (dimerization and phosphotransfer, residues 142-270) forms hexamers and octamers. NaCl treatment reduces the histidine kinase domain to tetramers and the DHp domain to hexamers, suggests the DHp domain senses NaCl. Autophosphorylates, possibly on His-161. Belongs to the chloroplast sensor kinase protein family.