The Hik2-Rre1 interaction acts as a two-component signaling system in filamentous cyanobacterium Anabaena PCC 7120

Photosynthetic organisms often rely on two-component regulatory system to adapt to environmental changes. This system is crucial for connecting external signals with the response mechanism by controlling gene expression, eventually allowing the organism to acclimatize to various stresses. Cyanobacteria, in particular, possess a large number of these two-component systems. Chloroplast Sensor Kinase (CSK) is a conserved histidine kinase present in all photosynthetic organisms. In the present study, Hik2, a CSK homolog found in the filamentous cyanobacterium Anabaena PCC 7120, was investigated to understand its role in the signaling mechanism of this organism. Recombinant Hik2 was found to undergo autophosphorylation on a conserved histidine residue, which remains unaffected by low salt concentrations but is slightly inhibited at elevated concentrations. Rre1 was identified as a potential partner for Hik2 through in silico analysis. Further experiments, including pull-down and surface plasmon resonance analysis, confirmed a physical interaction between Hik2 and Rre1. Interestingly, rapid dephosphorylation of Hik2 in the presence of Rre1 suggested a phosphotransfer from Phospho-Hik2 to its cognate partner Rre1. In silico analysis further identified probable heat-responsive regulons of Anabaena Rre1, suggesting the possible role of the Hik2-Rre1 interaction in the signaling mechanism of Anabaena PCC 7120. Overall, this study sheds light on the importance of the Hik2-Rre1 interaction in facilitating signaling processes in filamentous cyanobacteria, providing valuable insights into the acclimatization mechanisms of these photosynthetic organisms.