Two-component systems (TCSs) are ubiquitous signaling units found in prokaryotes. A TCS consists of a sensor histidine kinase and a response regulator protein as signal transducers. These regulatory systems mediate acclimation to various environmental changes by coupling environmental cues to gene expression. Hik2 is a sensor histidine kinase and its gene is found in all cyanobacteria. Hik2 is the homolog of Chloroplast Sensor Kinase (CSK), a protein involved in redox regulation of chloroplast gene expression during changes in light quality in plants and algae. Here we describe biochemical characterization of the signaling mechanism of Hik2 and its phosphotransferase activity. Results presented here indicate that Hik2 undergoes autophosphorylation on a conserved histidine residue, and becomes rapidly dephosphorylated by the action of response regulators Rre1 and RppA. We also show that the autophosphorylation of Hik2 is specifically inhibited by sodium ions.
Keywords: Chloroplast Sensor Kinase (CSK); Histidine Kinase 2; Synechocystis sp. PCC 6803; photosystem stoichiometry; redox regulator; redox sensor; salt stress; transcriptional control.