UC Berkeley

Photosynthesis encompasses the set of reactions that convert light into stored chemical energy. Photosynthetic organisms are the foundation of life, as they catalyze the assimilation of inorganic carbon (Ci) into the biosphere and are the ultimate source of nutrition on Earth.

Cyanobacteria are an ancient group of oxygenic photosynthetic prokaryotes and remain important primary producers today. The chloroplasts of all land plants can trace their ancestry to cyanobacteria. In many ways, such as the light reactions of photosynthesis that capture solar energy, the protein machinery of cyanobacteria is a representative, simplified form of that found in chloroplasts. In other ways, the cyanobacterial machinery is more complex as in the dark reactions that ultimately store solar energy as chemical energy by fixing CO2. To overcome inherent limitations in the principal CO2-fixing enzyme rubisco, cyanobacteria possess an elaborate integrated physiological strategy to increase the local CO2 concentration near rubisco and facilitate on-pathway catalysis. This physiological strategy, known as a CO2-concentrating mechanism (CCM), relies on energy-coupled Ci-transporters and encapsulation of rubisco in proteinaceous organelles known as carboxysomes. While some species of land plants possess CCMs, none resemble nor are nearly as productive as those found in cyanobacteria. Therefore, cyanobacteria are an ideal platform to study the basic workings of photosynthetic machinery and potentially provide insight on engineering strategies that may improve photosynthetic productivity in land plants of societal interest.

Here, we investigated the structure and function of protein complexes from cyanobacteria and other prokaryotes to elucidate their roles in photosynthesis and CCMs. First, we determined the structure of the NAD(P)H dehydrogenase-like complex, a key component in the light reactions, and develop putative models for its catalytic mechanism. Next, we discovered a new type of Ci-transporter we named the DABs accumulate bicarbonate (DAB) complex and characterize the complex through a variety of genetic, biochemical, and biophysical approaches. Finally, we determined the structure of simplified carboxysome shells that provide insight into the architecturally principles and assembly of native shell components.