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 V_max, 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 K_M 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