The cyanobacterium Synechocystis sp. PCC 6803 possesses multiple inorganic carbon (Ci) uptake systems that are regulated by Ci availability. The control mechanisms of these systems and their integration with other cell functions remain to be clarified. An analysis of the changes in global gene expression in response to Ci downshift and the inactivation of ndhR (sll1594), a LysR family regulator of Ci uptake is presented in this report. Mild Ci limitation (3% CO 2 (v/v) in air to air alone) induced a dramatic up-regulation of genes encoding both inducible CO 2 and HCO 3 ؊ uptake systems. An induction of ndhD5/ndhD6 and other genes in a probable transcriptional unit was observed, suggesting a function in inducible Ci uptake. The expression of slr1513 and sll1735, physically clustered with sbtA and ndhF3/ ndhD3/cupA, respectively, were also coordinated with upstream genes encoding the essential components for HCO 3 ؊ and CO 2 uptake. Ci limitation induced the regulatory genes slr1214, sll1292, slr1594, sigD, sigG, and sigH, among which slr1214, a two-component response regulator, showed the earliest induction, implying a role for the early response to Ci limitation. Opposite regulation of genes encoding the assimilation of carbon and nitrogen demonstrated a striking coordination of expression to balance C-and N-fluxes. The analyses revealed that ndhR inactivation up-regulated the expression of sbtA/sbtB, ndhF3/ndhD3/cupA/sll1735, and slr2006-13 including ndhD5 and ndhD6, indicating a vital role of this regulatory gene in both CO 2 and HCO 3 ؊ acquisition of the cyanobacterium. We therefore suggest that ndhR be renamed ccmR to better represent its broader regulatory characteristics.

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A sensor histidine kinase of Synechococcus sp. strain PCC7942, designated nblS, was previously identified and shown to be critical for the acclimation of cells to high-light and nutrient limitation conditions and to influence the expression of a number of light-responsive genes. The nblS orthologue in Synechocystis sp. strain PCC6803 is designated dspA (also called hik33). We have generated a dspA null mutant and analyzed global gene expression in both the mutant and wild-type strains under high-and low-light conditions. The mutant is aberrant for the expression of many genes encoding proteins critical for photosynthesis, phosphate and carbon acquisition, and the amelioration of stress conditions. Furthermore, transcripts from a number of genes normally detected only during exposure of wild-type cells to high-light conditions become partially constitutive in the low-light-grown dspA mutant. Other genes for which transcripts decline upon exposure of wild-type cells to high light are already lower in the mutant during growth in low light. These results suggest that DspA may influence gene expression in both a positive and a negative manner and that the dspA mutant behaves as if it were experiencing stress conditions (e.g., high-light exposure) even when maintained at near-optimal growth conditions for wild-type cells. This is discussed with respect to the importance of DspA for regulating the responses of the cell to environmental cues.Photosynthetic organisms have evolved intricate mechanisms for sensing and acclimating to environmental change. Parameters such as light quality, light intensity, and nutrient availability can modulate both the structure and the function of the photosynthetic machinery. Physiological and biochemical changes elicited by external cues include modification of lightharvesting complex (LHC) synthesis and degradation (6,12,27,29,38,45,65,75), changes in absorption and excitation energy transfer properties of LHC (11,18,28,59), and a modification of reaction center function (59, 80). Intracellular cues critical for controlling cellular processes during acclimation may reflect the cell's growth potential, cellular redox conditions, and/or accumulation of reactive oxygen species (52,77).Precise control over the fate of absorbed excitation energy is critical for cell viability during exposure of photosynthetic cells to excess excitation, since energized pigment molecules may trigger the production of damaging, reactive oxygen species (2). Over the short term, cells can dissipate excess absorbed excitation energy as heat by quenching excited pigment molecules in the LHC or by eliciting a state transition in which the LHC of photosystem II (PS II) directs its excitation energy to PS I, where quenching can occur. Over the long term, excess excitation may cause a dramatic reduction in the level of LHC. In cyanobacteria, a reduction in LHC size is reflected in reduced levels of transcripts encoding light-absorbing polypeptides (or phycobiliproteins) of the major LHC (or phycobilisomes). Several st...

Polymerase chain reaction-based mutageneses identify key transporters belonging to multigene families involved in Na+ and pH homeostasis of Synechocystis sp. PCC 6803 adapts to a wide range of adverse environments including high salinity (Joset et al., 1996). These characteristics make it a useful model for understanding stress tolerance in relation to oxygenic photosynthesis. Glucosylglycerol has been identified in Synechocystis sp. PCC 6803 as an osmoprotectant that contributes to its salt tolerance (Mikkat et al., 1996). On the other hand, the mechanisms acting against the ion toxicity of salt stress in the cyanobacterium remain to be elucidated because, not only osmotic pressure, but also ion effects account for its decrease in photosynthetic activity (Allakhverdiev et al., 1999). Na + /H+ antiporter activity appears to be involved in the intracellular Na + homeostasis of Synechococcus sp. PCC 6311 (Blumwald et al., 1984). However, salt stress depresses the activity and synthesis of Na + /H + antiporters in Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7942 (Allakhverdiev et al., 1999;. There is a dramatic loss of Na+ antiporter activity in Synechocystis sp. PCC 6803 in the presence of 1.0 M NaCl, although it can tolerate up to 1.2 M NaCl (Reed and Stewart, 1985;Allakhverdiev et al., 1999). Apart from Na + /H + antiporters, other membrane transporters may be involved in Na + homeostasis in this cyanobacterium. Both prokaryotic and eukaryotic cells possess primary Na + -ATPases, which extrude Na + directly (Gimmler, 2000). There are several potential Na + -ATPases in the Synechocystis sp. PCC 6803 genome, but no Na + -ATPase in any cyanobacterium has been definitively identified thus far (Ritchie, 1998). Bioenergetic analysis reveals that, under physiological ranges of pH, Na + -coupled secondary ion transport across membranes occurs in Synechococcus sp. PCC 7942 (Ritchie, 1992(Ritchie, , 1998, implying that there is an energyconsuming Na + efflux mechanism in cyanobacteria. Additionally, it has been well established that counteraction of Na + and other metal ions such as K + and Ca 2+ is also a determinant factor in Na + homeostasis. Therefore, it remains to be established whether or not the predicted energy-dependent Na + efflux protein(s) actually exist and the extent to which other cation ATPases may also affect Na + sensitivity of cyanobacteria. In addition to Na + homeostasis, Na + /H + antiporters are also involved in internal pH regulation . In cyanobacteria, light induces the transient acidification of culture medium in a process that /H+ antiporters (slr1727, sll0273, sll0689, slr1595 and slr0415) and seven cation ATPases (sll1614, sll1920, slr0671-72, slr0822, slr1507-08-09, slr1728-29 and slr1950) in the model cyanobacterium (http://www.kazusa.or.jp/cyano/cyano.html) were performed in this study relying on homologous recombination with mutagenenic fragments constructed using a fusion polymerase chain reaction (PCR) approach. The impacts of these gene knock-outs were evaluated in terms of N...