Carboxysome encapsulation of the CO2-fixing enzyme Rubisco in tobacco chloroplasts

A long-term strategy to enhance global crop photosynthesis and yield involves the introduction of cyanobacterial CO2-concentrating mechanisms (CCMs) into plant chloroplasts. Cyanobacterial CCMs enable relatively rapid CO2 fixation by elevating intracellular inorganic carbon as bicarbonate, then concentrating it as CO2 around the enzyme Rubisco in specialized protein micro-compartments called carboxysomes. To date, chloroplastic expression of carboxysomes has been elusive, requiring coordinated expression of almost a dozen proteins. Here we successfully produce simplified carboxysomes, isometric with those of the source organism Cyanobium, within tobacco chloroplasts. We replace the endogenous Rubisco large subunit gene with cyanobacterial Form-1A Rubisco large and small subunit genes, along with genes for two key alpha-carboxysome structural proteins. This minimal gene set produces carboxysomes, which encapsulate the introduced Rubisco and enable autotrophic growth at elevated CO2. This result demonstrates the formation of alpha-carboxysomes from a reduced gene set, informing the step-wise construction of fully functional alpha-carboxysomes in chloroplasts.