(60b) Engineering the Cyanobacterial Photosynthetic Electron Transport Chain to Improve Photosynthetic Efficiency.

Solar energy harvested by cyanobacteria is a primary source for energy. Therefore, it is imperative to increase its photosynthetic efficiency to secure our food supply. 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 (cytb6f), 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 under atmospheric CO2 showed that at low light (20 μE m−2 s−1), there was a 52% and 39% increase in cell density of the strain expressing PetD and Linker compared to the WT at day 12, respectively. Interestingly, this difference in growth at moderate light (80 μE m−2 s−1) was 34% and 15% for strain expressing PetD and Linker compared to WT at day 11. Experiments under atmospheric CO2 and high light (150 μE m−2 s−1) also demonstrated an extended growth pattern with 63% increase in biomass production for PetD and Linker strains compared to WT by day 20. Joliot type spectroscopy measures the activity of PSI’s ability to complete electron flow in the photosynthetic electron transport chain also highlights the PetD with a higher electron flow than both WT and the Linker at all light intensities (20, 45, 80, 150, 320 and 2050 μE m-2 s-1). The Linker strain further showed more rapid electron flow than WT only at light intensities of 20, 45, and 80 μE m-2 s-1. These findings confirm our hypothesis that in the engineered PetD and Linker strains, the electrons were channeled more effectively through the photosynthetic electron transport chain and thereby, improved photosynthetic efficiency which can be translated into higher growth rate.