Abstract
In general, histidine kinases (Hiks) perceive specific stimuli at the unique N-terminal signal-input domain (SID) and transmit the signals via the conserved C-terminal domain. The cyanobacterium Synechocystis sp. PCC 6803 has a Hik, Hik33, which is responsive to various stress conditions. However, the actual mechanism for signal perception has not been clarified. Hik33 has multiple subdomains in the SID, such as two transmembrane helices, a periplasmic loop, a HAMP and a PAS domain,. To investigate mechanisms of signal perception, we expressed a chimera Hik possessing the SID of Hik33 and the C-terminal transmitter domain of SphS, that is responsive to phosphate-deficient conditions, in cells deleted the sphS gene. The chimera Hik responded to salt stress conditions and regulated phoA gene for alkaline phosphatase which is ordinarily regulated by SphS. Although a removal of both HAMP and PAS domains from the chimera Hik lost the kinase ability, a removal of the HAMP domain maintained the activity of chimera Hik and responsiveness to salt stress conditions. These results indicated that the HAMP domain is not essential for response to salt stress conditions.
