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. 2018 Sep 3;9:3570. doi: 10.1038/s41467-018-06044-0

Fig. 1.

Fig. 1

Cyanobacterial CCM components for improved photosynthesis. Cyanobacterial CCMs (a) use bicarbonate (HCO3) and CO2 pumps on the plasma and thylakoid membranes, respectively, to elevate cytosolic HCO3. This HCO3 pool is utilized by icosahedral-shaped Rubisco microcompartments called carboxysomes (yellow icosahedron), where HCO3 is converted to CO2 by localized carbonic anhydrase (CA) and accumulates due to resistive CO2 efflux. The carboxysome encapsulates the cell’s complement of a high-catalytic-rate Rubisco, which operates at close to Vmax, converting ribulose-1,5-bisphosphate (RuBP) to 3-phosphoglycerate (3-PGA) within the Calvin cycle. This mechanism leads to efficient photosynthesis and reduced nitrogen allocation compared to C3 plants. A strategy to improve C3 photosynthesis in plant cells (b, represented here as rectangular structures containing dual-membrane chloroplasts) using the cyanobacterial CCM recommends independent transfer of carboxysomes containing their cognate Rubisco to the chloroplast stroma (i) and HCO3 pumps (ii) to the chloroplast inner-envelope membrane. Successful transfer of HCO3 transporters alone (ii) should generate a moderately elevated stromal HCO3 pool (indicated by the change in colour shading of the chloroplast) and has a predicted photosynthetic improvement of 9%15 due to the resulting net elevation of CO2 near Rubisco. Expression of functional carboxysomes (i), or just their cognate Rubisco, in the chloroplast stroma should lead to a high CO2-requiring (HCR) phenotype due to the characteristically high KM and low specificity for CO2 of carboxysomal Rubiscos and absence of an elevated HCO3 or CO2 pool. However, in combination (iii), generation of high stromal HCO3 pool in the presence of functional carboxysomes, with stromal CA eliminated, is predicted to generate a stromal HCO3 concentration approaching 5 mM16 and to increases in CO2 fixation and yield of up to 60%15. c Carboxysomes of Cyanobium PCC7001, used in this study, consist of many thousands of polypeptides, arranged in an icosahedral structure. In this model, a single layer of shell-bound Rubisco (CbbLS, green) is shown, with carboxysomal CA (orange). CsoS2 (yellow/brown) interlinks Rubisco and the shell made predominantly of CsoS1A hexamers (light blue). These and ancillary shell proteins (CsoS1D and CsoS1E, dark blue) enable substrate transport via central pores. Pentameric vertex proteins (CsoS4AB, purple) complete the structure