(499e) Engineering the Cyanobacterial Photosynthetic Electron Transport Chain to Improve Photosynthetic Efficiency

Solar energy harvested by photosynthetic systems including plants, algae, and cyanobacteria is a primary source for energy. Therefore, it is imperative to increase the photosynthetic efficiency of the photoautotrophic microorganisms to secure our food supply for today and the future. To this end, the cyanobacterium Synechocystis sp. PCC 6803 (hereafter, Synechocystis) serves as a perfect model organism as it is easy to engineer and shares all energy bottlenecks ubiquitous to photosynthesis and respiration in plant cells. In this study, we designed two distinct strategies to engineer the photosynthetic electron transport chain in Synechocystis to increase the photosynthetic efficiency. The first approach was directed towards linking the photosynthetic complexes photosystem I (PSI) and cytochrome bf (cytbf), to bring them in a closer proximity and thereby, reduce electron loss. We performed computational 3D protein analysis to identify suitable proteins to link cyt b6f and PSI. Comprehending the protein orientations and interpeptide distances PetA from cyt b6f (C-terminus) and PsaA from PSI (N-terminus) were identified as suitable candidates to be linked. Following these strains expressing the linked proteins were constructed for further experiments.

In the second approach to increase the membrane density of cyt b6f complex and thereby reduce the proximity between cyt b6f with PSI and PSII, strains were constructed to overexpress the major proteins PetA, PetB, PetC, and PetD. The hypothesis was that overexpressing one of the major proteins would result in increasing the density of cyt b6f by leveraging unknown endogenous regulatory systems. Comparative growth studies of the engineered strains with wild type showed that at low light (20 μE m^−2 s^−1), there was a 48% increase in growth of the strain expressing PetD compared to WT. Additional studies are underway to get all the strains segregated as well as to test their growth under low, medium, and high light conditions as well as by varying CO₂. In addition, to understand the molecular cause for the increase in photosynthetic yields we will also conduct further studies by using PAM fluorimetry, 77K spectroscopy, and also by tracking the oxygen evolution rates.