Photorespiratory 2-phosphoglycolate (2PG) metabolism is essential for photosynthesis in higher plants but thought to be superfluous in cyanobacteria because of their ability to concentrate CO 2 internally and thereby inhibit photorespiration. Here, we show that 3 routes for 2PG metabolism are present in the model cyanobacterium Synechocystis sp. strain PCC 6803. In addition to the photorespiratory C2 cycle characterized in plants, this cyanobacterium also possesses the bacterial glycerate pathway and is able to completely decarboxylate glyoxylate via oxalate. A triple mutant with defects in all 3 routes of 2PG metabolism exhibited a high-CO 2-requiring (HCR) phenotype. All these catabolic routes start with glyoxylate, which can be synthesized by 2 different forms of glycolate dehydrogenase (GlcD). Mutants defective in one or both GlcD proteins accumulated glycolate under high CO 2 level and the double mutant ⌬glcD1/⌬glcD2 was unable to grow under low CO2. The HCR phenotype of both the double and the triple mutant could not be attributed to a significantly reduced affinity to CO2, such as in other cyanobacterial HCR mutants defective in the CO2-concentrating mechanism (CCM). These unexpected findings of an HCR phenotype in the presence of an active CCM indicate that 2PG metabolism is essential for the viability of all organisms that perform oxygenic photosynthesis, including cyanobacteria and C3 plants, at ambient CO 2 conditions. These data and phylogenetic analyses suggest cyanobacteria as the evolutionary origin not only of oxygenic photosynthesis but also of an ancient photorespiratory 2PG metabolism. I t is well established that the photorespiratory C2 pathway, whereby 2-phosphoglycolate (2PG) is metabolized (1), is essential for photosynthesis in the majority of plants (2). In contrast, the functioning of the C2 pathway and its importance are still under discussion for cyanobacteria. These organisms were the first to have evolved oxygenic photosynthesis, and endosymbiotic engulfment of an ancient cyanobacterium led to the evolution of plant chloroplasts (3). In cyanobacteria, as in C3 plants, the primary carbon fixation is catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Ribulose 1,5-bisphosphate reacts with either CO 2 , leading to the formation of 2 molecules of 3-phosphoglycerate (3PGA), or O 2 , generating 3PGA and 2PG. The latter compound is toxic to plant metabolism because it inhibits distinct steps in the carbon-fixing Calvin-Benson cycle (4, 5). Therefore, plants employ the socalled photorespiratory glycolate pathway (or C2 cycle), which degrades 2PG and converts 2 molecules of 2PG into 1 molecule each of 3PGA, CO 2 , and NH 4 ϩ (1, 6, 7). In a typical C3 plant, the ammonium is refixed at the expense of ATP, and 25% of the carbon entering the path is released as CO 2 . Generally, the photorespiratory cycle is indispensable for C3 plants, because mutations in single steps of the C2 cycle resulted in high-CO 2 -requiring (HCR) phenotypes (2,(8)(9)(10).In contrast to plants, ear...
In Synechocystis sp. strain PCC 6803, over 450 genes are upregulated following transfer of the cells from a high (1-5% CO(2) in air, HC) to a low level of CO(2) (as in air or lower, LC). This includes sbtA, ndhF3 and cmpA involved in inorganic carbon (Ci) uptake. Earlier studies implicated NdhR in the regulation of LC-induced genes but there are indications that additional components are involved. Following extraction of proteins from cells grown under HC and (NH4)(2)SO(4) fractionation, we have identified LexA and two AbrB-like proteins, Sll0359 and Sll0822, which bind to a fragment of the sbtA promoter. Using extracts prepared from LC-grown cells, Sll0822 did not bind to the sbtA promoter despite its presence in the cells, suggesting that it may serve as a repressor of LC-induced genes. This is supported by the fact that sbtA, ndhF3 and cmpA normally expressed only under LC in the wild-type are transcribed under both HC and LC in a Deltasll0822 mutant. When grown under HC this mutant exhibits an elevated apparent photosynthetic affinity to Ci, typically observed in the wild-type only under LC. Clearly, expression of genes essential for Ci uptake was sufficient to raise the apparent photosynthetic affinity for external Ci.
Using a polyphasic approach, we examined the presence of Archaea in the Gulf of Aqaba, a warm marine ecosystem, isolated from major ocean currents and subject to pronounced seasonal changes in hydrography. Catalyzed reported deposition FISH analyses showed that Archaea make up to >20% of the prokaryotic community in the Gulf. A spatial separation between the two major phyla of Archaea was observed during summer stratification. Euryarchaeota were found exclusively in the upper 200 m, whereas Crenarchaeota were present in greater numbers in layers below the summer thermocline. 16S rRNA gene-based denaturing gradient gel electrophoresis confirmed this depth partitioning and revealed further diversity of Crenarchaeota and Euryarchaeota populations along depth profiles. Phylogenetic analysis showed pelagic Crenarchaeota and Euryarchaeota to differ from coral-associated Archaea from the Gulf, forming distinct clusters within the Marine Archaea Groups I and II. Endsequencing of fosmid libraries of environmental DNA provided a tentative identification of some members of the archaeal community and their role in the microbial community of the Gulf. Incorporation studies of radiolabeled leucine and bicarbonate in the presence of different inhibitors suggest that the archaeal community participates in autotrophic CO(2) uptake and contributes little to the heterotrophic activity.
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