In previous studies, we characterized five histidine kinases (Hiks) and the cognate response regulators (Rres) that control the expression of ϳ70% of the hyperosmotic stress-inducible genes in the cyanobacterium Synechocystis sp. PCC 6803. In the present study, we screened a gene knock-out library of Rres by RNA slotblot hybridization and with a genome-wide DNA microarray and identified three Hik-Rre systems, namely, Hik33-Rre31, Hik10-Rre3, and Hik16-Hik41-Rre17, as well as another system that included Rre1, that were involved in perception of salt stress and transduction of the signal. We found that these Hik-Rre systems were identical to those that were involved in perception and transduction of the hyperosmotic stress signal. We compared the induction factors of the salt stress-and hyperosmotic stress-inducible genes that are located downstream of each system and found that these genes responded to the two kinds of stress to different respective extents. In addition, the Hik33-Rre31 system regulated the expression of genes that were specifically induced by hyperosmotic stress, whereas the system that included Rre1 regulated the expression of one or two genes that were specifically induced either by salt stress or by hyperosmotic stress. Our observations suggest that the perception of salt and hyperosmotic stress by the Hik-Rre systems is complex and that salt stress and hyperosmotic stress are perceived as distinct signals by the Hik-Rre systems.Responses to salt stress and hyperosmotic stress have been investigated in prokaryotes, fungi, and plants. However, there is some confusion in the literature because salt stress and hyperosmotic stress have been regarded both as equivalent and as distinct stimuli (1-4). In Arabidopsis thaliana, both salt stress due to 0.1 M NaCl and hyperosmotic stress due to 0.2 M mannitol regulate the expression of not only the same set of genes but also of different sets of genes (4). In the cyanobacterium Synechocystis sp. PCC 6803 (hereafter, Synechocystis), it is clear that there are major differences between the sets of genes that respond to salt stress due to 0.5 M NaCl and hyperosmotic stress due to 0.5 M sorbitol (3). Moreover, the cytoplasmic volume of Synechocystis decreases by ϳ70% of the original volume within 10 min when cells are exposed to 0.5 M sorbitol, but the decrease in cytoplasmic volume is only 30% with 0.5 M NaCl (3). Although the responses to hyperosmotic stress and salt stress are different in terms of gene expression and changes in cytoplasmic volume, recent studies have demonstrated that the same histidine kinases (Hiks), 1 such as Hik33, Hik34, and Hik16, might be involved in the perception of salt and hyperosmotic stress (5, 6).In Synechocystis, several Hiks that are paired with specific response regulators (Rres) have been identified as regulators of the response to hyperosmotic stress (6). A specific Hik senses hyperosmotic stress, and it seems likely that the signal is transferred to the cognate Rre by transfer of a phosphate group from the histidine...

Microorganisms respond to hyperosmotic stress via changes in the levels of expression of large numbers of genes. Such responses are essential for acclimation to a new osmotic environment. To identify factors involved in the perception and transduction of signals caused by hyperosmotic stress, we examined the response of Synechocystis sp. PCC 6803, which has proven to be a particularly useful microorganism in similar analyses. We screened knockout libraries of histidine kinases (Hiks) and response regulators (Rres) in Synechocystis by DNA microarray and slot-blot hybridization analyses, and we identified several two-component systems, which we designated Hik-Rre systems, namely, Hik33-Rre31, Hik34-Rre1, and Hik10-Rre3, as well as Hik16-Hik41-Rre17, as the transducers of hyperosmotic stress. We also identified Hik2-Rre1 as a putative additional twocomponent system. Each individual two-component system regulated the transcription of a specific group of genes that were responsive to hyperosmotic stress.

SummaryOxidative stress caused by reactive oxygen species and, in particular, to hydrogen peroxide (H 2 O 2 ) has a major impact on all biological systems, including plants and microorganisms. We investigated the H 2 O 2 -inducible expression of genes in the cyanobacterium Synechocystis sp. PCC 6803 using genome-wide DNA microarrays. Our systematic screening of a library of mutant lines with defects in histidine kinases (Hiks) by RNA slot-blot hybridization and DNA-microarray analysis suggested that four Hiks, namely, Hik33, Hik34, Hik16 and Hik41, are involved in the perception and transduction of H 2 O 2 signals that regulate the gene expression of 26 of the 77 H 2 O 2 -inducible genes with induction factors higher than 4.0. Among the four Hiks, Hik33 was the main contributor and was responsible for 22 of the 26 H 2 O 2 -inducible genes under the control of the Hiks. By contrast to Hik33, PerR encoding putative peroxide-sensing protein is involved in the regulation of only nine H 2 O 2 -inducible genes.

Arthrospira platensis is a cyanobacterium that is extensively cultivated outdoors on a large commercial scale for consumption as a food for humans and animals. It can be grown in monoculture under highly alkaline conditions, making it attractive for industrial production. Here we describe the complete genome sequence of A. platensis C1 strain and its annotation. The A. platensis C1 genome contains 6,089,210 bp including 6,108 protein-coding genes and 45 RNA genes, and no plasmids. The genome information has been used for further comparative analysis, particularly of metabolic pathways, photosynthetic efficiency and barriers to gene transfer.

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Arthrospira (Spirulina) platensis is a well-known commercial cyanobacterium that is used as a food and in feed supplements. In this study, we examined the physiological changes and whole-genome expression in A. platensis C1 exposed to high temperature. We found that photosynthetic activity was significantly decreased after the temperature was shifted from 35°C to 42°C for 2 h. A reduction in biomass production and protein content, concomitant with the accumulation of carbohydrate content, was observed after prolonged exposure to high temperatures for 24 h. Moreover, the results of the expression profiling in response to high temperature at the designated time points (8 h) revealed two distinct phases of the responses. The first was the immediate response phase, in which the transcript levels of genes involved in different mechanisms, including genes for heat shock proteins; genes involved in signal transduction and carbon and nitrogen metabolism; and genes encoding inorganic ion transporters for magnesium, nitrite and nitrate, were either transiently induced or repressed by the high temperature. In the second phase, the long-term response phase, both the induction and repression of the expression of genes with important roles in translation and photosynthesis were observed. Taken together, the results of our physiological and transcriptional studies suggest that dynamic changes in the transcriptional profiles of these thermal-responsive genes might play a role in maintaining cell homeostasis under high temperatures, as reflected in the growth and biochemical composition, particularly the protein and carbohydrate content, of A. platensis C1.

Identical Hik-Rre systems are involved in perception and transduction of salt signals and hyperosmotic signals but regulate the expression of individual genes to different extents in Synechocystis.