Sensors of Environmental Stress

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Histidine kinase (Hik) and response regulator (Rre) constitute the twp-component system that is a signal perception/transduction pathway in prokaryotes and some eukaryotes. The cyanobacterium Synechocystis sp. PCC 6803, which performs oxygenic photosynthesis like plants, contains 47 genes for Hiks and 45 genes for Rres. We mutated these genes successfully, except three genes for Hiks and three genes for Rres, and constructed mutant libraries of Hiks and Rres. By screening the libraries by genome-wide expression of genes under various stress conditions with cDNA microarrays, we obtained the following results.Hyperosmotic stress, due to 0.5 M sorbitol, induced the expression of 246 genes. Screening of the above-mentioned mutant libraries with respect to the hyperosmotic stress-inducible expression of genes by the cDNA microarray method revealed that 60% of the hyperosmotic stress-inducible genes are under control of five two-component systems, namely, Hik33/Rre31, Hik16/Hik41/Rre17, Hik34/Rre1, Hik2/Rre1, and Hik10/Rre3. Remaining 40% of the hyperosmotic stress-inducible genes are under control of as-yet unidentified signal-transducing pathways.Salt stress, due to 0.5 M NaCl, induced the expression of 294 genes. Screening of the above-mentioned mutant libraries with respect to the salt stress-inducible expression of genes by the cDNA microarray method revealed that 70% of the salt stress-inducible genes are under control five two-component systems as in the case of hyperosmotic stress-inducible genes. However, genes under the individual Hik/Rre two-component systems are different between the salt stress and hyperosmotic stress. Remaining 30% of the salt stress-inducible genes are under control of as-yet unidentified signal-transducing pathways.