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Powering cells with light: the synthetic biology of photosynthesis

Final Report Summary - SYNTHPHOTO (Powering cells with light: the synthetic biology of photosynthesis)

The scientific problem to be addressed. Life on Earth is powered by the solar energy absorbed by billions of tonnes of chlorophylls made by plants, algae and bacteria. Inside each photosynthetic cell millions of chlorophyll molecules are attached to proteins so they can collect and use energy from the Sun. We aimed to find out how the chlorophylls and proteins are made, and how they are put together to capture light and convert it into ATP, which powers the thousands of chemical reactions that enable the cells to grow and divide. This knowledge is important to us all, not just because capturing and using solar energy fuels life, but it also holds the secret of designing and making devices that one day could give us clean, unlimited energy from sunlight.

The SYNTHPHOTO project comprised two interlinked sections: SP1 investigates how newly-synthesised chlorophyll pigments feed into the membrane-embedded assembly machinery and form growing photosynthetic units. SP2 harnesses this knowledge for the design and construction of new biological and bioinspired light-gathering and energy-trapping systems.
Outcomes of SYNTHPHOTO. Overall, 66 papers were published, including many in high-profile journals such as Plant Cell, Cell, Nature, Nature Plants, Science Advances, Nature Communications, ACS Nano, eLife, PNAS USA.

SP1 Elucidation of the mechanisms of assembly of photosynthetic complexes. We studied the enzymes that make chlorophyll (Chl), and the first assembly of all the Chl biosynthesis reactions in a foreign host establishes a platform for engineering photosynthesis. We used atomic force microscopy (AFM), which uses a nanoscale probe to ‘feel the bumps’ of proteins in biological membranes, to image assembly intermediates of photosynthetic complexes, and in the model cyanobacterium Synechocystis we examined the role of Chls in photosystem assembly. We swapped the genes for cyanobacterial and plant Chl biosynthesis enzymes to examine conserved features of the assembly apparatus.
Supercomputing yielded a 100 million-atom model of the bacterial photosynthetic membrane that can simulate all steps, from ‘photon to ATP’. The photosynthetic membranes of cyanobacteria were mapped by AFM, including marine Prochlorococcus, the most abundant phototroph on Earth. We estimate that the combined surface area of energy-absorbing Prochlorococcus membranes is 28 times the surface area of the Earth. We also showed how the cyanobacterial photosynthetic apparatus is extensively remodelled during adaptation to growing in far-red light.

SP2. Designing and building new photosynthetic functions.
We collaborated with ultrafast spectroscopists to examine the flow of solar energy in a living photosynthetic cell, for the first time, and used cryo-electron microscopy to determine the molecular structure of a photosynthetic complex that harvests and traps infrared light. We also invented a new form of AFM to measure the forces involved in allowing electrons to move between proteins in respiration and photosynthesis.
Protein engineering was used to create a photosynthetic complex with extra light absorbing power that allowed the new organism to grow faster, and we worked with a collaborator on bacterially-derived reprogrammable SimCells lacking a native chromosome, for future synthetic biology work.
We invented nanoscale surface fabrication approaches so we could assemble artificial photosystems in a defined and predictable manner on gold or silicon, like biologically-based computer chips. These biohybrid arrays were built using photosynthetic components from bacteria and plants. AFM and a home-built fluorescence lifetime imaging microscope were used to show that these arrays could absorb light energy and transfer it among the surface-bound molecules.