![]() ![]() This mechanism leads to efficient photosynthesis and reduced nitrogen allocation compared to C 3 plants. The carboxysome encapsulates the cell’s complement of a high-catalytic-rate Rubisco, which operates at close to V max, converting ribulose-1,5-bisphosphate (RuBP) to 3-phosphoglycerate (3-PGA) within the Calvin cycle. This HCO 3 − pool is utilized by icosahedral-shaped Rubisco microcompartments called carboxysomes (yellow icosahedron), where HCO 3 − is converted to CO 2 by localized carbonic anhydrase (CA) and accumulates due to resistive CO 2 efflux. Cyanobacterial CCMs ( a) use bicarbonate (HCO 3 −) and CO 2 pumps on the plasma and thylakoid membranes, respectively, to elevate cytosolic HCO 3 −. When the intracellular HCO 3 − pool is elevated, a high CO 2 environment can be generated inside the carboxysome, overcoming this low specificity and enabling rapid CO 2 fixation with reduced inhibition by oxygen 14.Ĭyanobacterial CCM components for improved photosynthesis. Cyanobacterial carboxysomes possess high-catalytic-turnover, but low-CO 2-specificity Rubisco enzymes 1. Physiological evidence 11 and mathematical models 12, 13 suggest carboxysomes resist CO 2 efflux, resulting in concentration of CO 2 around Rubisco. The carboxysome’s outer protein shell enables diffusional influx of HCO 3 − and RuBP, where the former is converted to CO 2 by a localized carbonic anhydrase (CA). This HCO 3 − pool is then utilized by subcellular micro-compartments called carboxysomes, which encapsulate the cell’s complement of Rubisco 10. The cyanobacterial CCM is a single-cell, bipartite system that first generates a high intracellular bicarbonate (HCO 3 −) pool through action of membrane-bound inorganic carbon (C i) transporters and CO 2-converting complexes 7, 8, 9 (Fig. A suggested approach to increase CO 2 fixation, minimize water-loss and decrease investment in Rubisco is to translate essential components of the cyanobacterial CO 2-concentrating mechanism (CCM) into C 3 crops 4, 5, 6. These latter inefficiencies are driven by passive acquisition of CO 2 from the air (leading to water loss via open stomata) and by large investment in Rubisco (up to 50% of leaf protein) to overcome its poor kinetics 3. However, Rubisco-mediated CO 2 fixation in C 3 chloroplasts is catalytically slow, competitively inhibited by oxygen 1 and, from an agricultural stand-point, makes inefficient use of water and combined nitrogen 2. Photosynthetic CO 2 fixation via ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the primary input of carbon into crop biomass. This result demonstrates the formation of α-carboxysomes from a reduced gene set, informing the step-wise construction of fully functional α-carboxysomes in chloroplasts. ![]() This minimal gene set produces carboxysomes, which encapsulate the introduced Rubisco and enable autotrophic growth at elevated CO 2. We replace the endogenous Rubisco large subunit gene with cyanobacterial Form-1A Rubisco large and small subunit genes, along with genes for two key α-carboxysome structural proteins. Here we successfully produce simplified carboxysomes, isometric with those of the source organism Cyanobium, within tobacco chloroplasts. To date, chloroplastic expression of carboxysomes has been elusive, requiring coordinated expression of almost a dozen proteins. Cyanobacterial CCMs enable relatively rapid CO 2 fixation by elevating intracellular inorganic carbon as bicarbonate, then concentrating it as CO 2 around the enzyme Rubisco in specialized protein micro-compartments called carboxysomes. A long-term strategy to enhance global crop photosynthesis and yield involves the introduction of cyanobacterial CO 2-concentrating mechanisms (CCMs) into plant chloroplasts. ![]()
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