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Cells use compartmentalization of enzymes as a strategy to regulate metabolic pathways and increase their efficiency. The α- and β-carboxysomes of cyanobacteria contain ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)—a complex of eight large (RbcL) and eight small (RbcS) subunits—and carbonic anhydrase^{, –}. As HCO₃⁻ can diffuse through the proteinaceous carboxysome shell but CO₂ cannot, carbonic anhydrase generates high concentrations of CO₂ for carbon fixation by Rubisco. The shell also prevents access to reducing agents, generating an oxidizing environment^{, –}. The formation of β-carboxysomes involves the aggregation of Rubisco by the protein CcmM, which exists in two forms: full-length CcmM (M58 in Synechococcus elongatus PCC7942), which contains a carbonic anhydrase-like domain followed by three Rubisco small subunit-like (SSUL) modules connected by flexible linkers …