Abstract - Improvement of yield potential in global food crops has hit a breeding road-block which could be solved through genetic engineering. One promising strategy is the introduction of cyanobacterial CO2-concentrating-mechanisms (CCMs) into plant chloroplasts to enhance photosynthesis and yield. The cyanobacterial CCM actively accumulates inorganic carbon (Ci) as the relatively membrane-impermeable bicarbonate ion in the cytoplasm, reaching concentrations up to 1000-fold that of the external water environment. This concentrated bicarbonate pool fuels the production of a high concentration of CO2 within specialized micro-compartments called carboxysomes, where the cyanobacterial Rubisco resides. By elevating localized CO2 concentrations around this notoriously slow and promiscuous enzyme, Rubisco can achieve rates of CO2 catalysis at or near a catalytic maximum which far exceeds that of typical C3 plants. Mathematical modelling of the proposed inclusion of a CCM into C3 plants suggests improvements in photosynthesis and yield by up to 60%, making enormous theoretical gains in future global food security. Nonetheless, the complexity of the components which drive and operate within the CCM make this a major technical challenge. Apart from considerable difficulty and effort to build functional Ci transporters into C3 chloroplasts, the carboxysome presents a complex and difficult engineering task. Carboxysomes of different cyanobacterial species range in size from 100 – 1000 MDa and consist of between nine and 13 different polypeptides, including Rubisco large and small subunits, with dramatically different stoichiometries. They are icosahedral structures of ~100 – 600 nm in diameter with complex surface-shell-protein, and internal-protein structure, representing a class of what have become known as bacterial micro-compartments, or prokaryotic organelles. I will present current progress and future goals in the step-wise construction of fully-functional α-carboxysomes in chloroplasts as we progress toward the improvement of crop plant performance.
Biography - Ben Long is a biochemist and plant biologist at RSB with a background in cyanobacterial (blue-green algal) physiology. His scientific interest centres on the carboxysome; a structural compartment which encapsulates cyanobacterial Rubisco, the primary enzyme of CO2 fixation. His principal areas of research are the structural interactions which make up carboxysomes from two convergently evolved forms of this bacterial microcompartment, and how this information can be used to generate functional carboxysomes inside C3 plant chloroplasts. Ben came to RSBS in 2003 and worked on β-carboxysome structure with Dean Price and Murray Badger. After a postdoc position with Owen Atkin working on the respiratory capacity of photosynthetic and non-photosynthetic leaf cell types, Ben returned to work on carboxysomes as a part of the RIPE Project, a Bill and Melinda Gates Foundation program tasked with improving the photosynthetic performance of crop plants in developing nations.