Studies on low-temperature sensors and the mechanisms of acclimation to low-temperature

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(1) The cyanobacterium Synechocystis sp. PCC 6803 contains 44 genes for histidine kinases (Hiks) and 42 genes for response regulators (Rres). We individually inactivated all of these signal transducers and generated knockout libraries. By screening these libraries by cold-inducible expression of genes with genome-wide DNA microarrays we identified Hik33 as only one Hik for cold sensing and Rre 26 and Rre 31 as response regulators located downstreem of Hik33 in the cold signal transduction pathway.(2) We identified Hik33, which we previously identified as a cold sensor, as a component involved in the signal pathways of hyperosmotic, cold, strong-light and oxidative stress. These findings suggest that Hik33 itself is not specific to the individual stress and that the signal transduction pathways which include Hiks and Rres are more complicated than the simple two-component systems which have been established previously. It is very likely that Hik33 cooperates with other factors that give the specificity to individual stress. Identifications of such factors will be an important subject of further study.(3) We demonstrated possibilities that some Ser/Thr protein kinases are involved in the cold-signal transduction pathway and that DNA supercoiling may directly sense the cold signals.