Functional analysis of the N-terminal region of an essential histidine kinase, Hik2, in the cyanobacterium Synechocystis sp. PCC 6803.

Histidine kinases are sensory proteins involved in the perception of environmental changes. Here, we characterized one of three essential histidine kinases, Hik2, in the cyanobacterium Synechocystis sp. PCC 6803 by constructing a fused sensor, Hik2n-Hik7c, which has the signal input domain of Hik2 and the kinase domain of the phosphate-deficiency sensor Hik7. The coding region of the hik7 gene was replaced with the fused sensor to evaluate the signalling activity in vivo as the activity of alkaline phosphatase (AP), which is regulated by Hik7. Cells expressing Hik2n-Hik7c had weak AP activities under standard growth conditions. Saline stress by NaCl induced AP activity in a dose-dependent manner. Analysis of the effects of several salt compounds on induction of AP activity indicated that Hik2n-Hik7c responded to Cl concentration. Amino acid substitution in the signal input domain of Hik2 resulted in loss of this responsiveness. These results suggest that the signal input domain of Hik2 responds to environmental Cl concentration in Synechocystis.