Microbial Control of Carbon Fixation

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Approximately one third of all the photosynthesis on earth is performed by aquatic bacteria. In the absence of membrane-bound compartments inside their cells, these organisms create a favorable environment for carbon-fixing enzymes by tightly packaging them inside protein compartments called carboxysomes. Despite the importance of carboxysomes to the global carbon cycle and their potential to serve as a platform for synthetic biology, the carboxysome assembly process is still poorly understood. This research will elucidate the molecular mechanisms specifying the size, number, and internal environment inside carboxysomes. It will blend biochemical approaches, synthetic biology, and fluorescence microscopy to explore the assembly process of whole carboxysomes over time, and to place this process in the context of cellular physiology. An understanding of the basic biological mechanisms by which carboxysomes are formed will lay the groundwork for future engineering of these compartments. Toward this end, this project will further the NSF's mission of expanding science and engineering research potential, whether by improving on natural carbon fixation or building nanoscale factories for other sensitive chemical reactions within living cells. This project will provide also an ideal interdisciplinary educational and training program. More broadly, the ultimate goal of the carboxysome work is to improve the engineering of novel functionalities. Such a synthetic life-like system could be endowed with the ability to sequester and contain reactions otherwise incompatible with the bacterial cytoplasm, or improve upon carbon fixation to create more efficient, living remediators of carbon dioxide. Reducing the levels of atmospheric carbon dioxide, a major greenhouse gas, would forestall climate changes, and therefore offer major societal benefit.As atmospheric carbon dioxide levels are approaching 400 ppm, understanding biological mechanisms of carbon fixation is becoming increasingly important. A large fraction of carbon dioxide is remediated by photosynthetic cyanobacteria, which concentrate the machinery required to fix carbon into proteinaceous microcompartments called carboxysomes. In Synechococcus elongatus, these organelles, are composed of a layer of shell proteins surrounding a matrix of the enzymes RuBisCO and carbonic anhydrase. Despite their importance to the global carbon cycle, the factors controlling assembly of these complex protein machines are poorly understood. Dr. Pamela Silver and her group at the Harvard Medical School will employ biochemical reconstitution, genetic perturbations, and quantitative live-cell microscopy to study the molecular basis and metabolic consequences of the spatial and temporal organization of carboxysome biogenesis.