Cryo-EM Studies of Macromolecular Complexes from Photosynthetic Organisms

Abstract: Plants, algae, and cyanobacteria convert light energy into chemical energy through the process of photosynthesis, fueling the planet and making life as we know it possible. Photosystem I (PSI) is one of the main photosynthetic complexes, responsible for this process. PSI uses the energy of light to transfer electrons from the soluble electron carrier plastocyanin, on the lumenal site of the thylakoid membrane, to ferrodoxin, on the stromal site of the membrane. Thus, playing a key role in the light dependent reactions. In order to survive many photosynthetic organisms need to be able to adapt to fluctuations in light and have adapted their photosynthetic machinery accordingly. In recent years many advances have been made in electron cryo-microscopy, making it possible to visualize many previously elusive photosynthetic complexes. This has brought a wealth of information on the structural adaptations of PSI.In plants and algae, PSI is hosted by the chloroplast, a specialized organelle that houses the photosynthetic reactions. In the chloroplast, key components of PSI are synthesized by the chloroplasts own translation machinery: the chloroplast ribosome. Translation in the chloroplast is remarkable as it has to synchronize translation in two different genetic compartments as well as adapt to fluctuations in light. A glimpse of how this machinery has evolved to be able to fulfill all of these duties can be obtained from its three dimensional structure and its chloroplast specific features. However, despite all this structural information providing valuable clues as to the functioning of these systems, there are still many aspects of how they play a role that still remain unknown.