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♦ See referenced article, J. Biol. Chem. 2010, 285, 19833–19841
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a critical photosynthetic enzyme and an attractive target for engineering; improving this enzyme's activity would enhance CO2 fixation and lead to increased agricultural productivity, whether for food, fiber, or energy. However, some substantial obstacles to Rubisco engineering exist, notably the lack of an effective biological system for the expression and assembly of hybrid enzymes that can be used to study the relative roles and contributions of the small and large subunits. In this Paper of the Week, though, Todor Genkov and colleagues describe the generation of a Chlamydomonas model where the native small subunits are replaced with ones from Arabidopsis, spinach, and sunflower. All three hybrid enzymes assembled correctly and displayed a slightly increased CO2/O2 specificity relative to native Chlamydomonas Rubisco while retaining near-normal carboxylation Vmax values. However, despite the presence of catalytically proficient Rubisco, these hybrid strains showed reduced photosynthetic growth, due to the lack of pyrenoids, the chloroplast sites where CO2 is concentrated for optimal photosynthesis. Together, these results suggest that the Rubisco small subunits contribute significantly to overall Rubisco activity, both by directly affecting catalytic performance and indirectly enhancing efficiency by targeting the enzyme to the algal pyrenoid.