A complementary photosystem for proton pumping in Synechocystis PCC 6803

Natural photosynthesis makes only limited use of the available photons, the so-called photosynthetically active region (PAR). In addition, part of these photons are used to drive ATP synthesis for the maintenance and growth of the organism and the concentration of CO2. We see an opportunity to make a fundamental transition towards more efficient photosynthesis by adding a 3rd photosystem, nicknamed photosystem 3 (PS3), since it complements the existing photosystems 1 and 2 (PS1 and PS2).

Synthetic biology will be used to implement this new photosystem that should serve to exploit the photons > 700 nm, outside the PAR, for modulating the proton gradient across the membrane. In this way the additional energy input will be used to produce additional ATP that can be used for metabolic purposes and CO2 concentration. This would allow the decoupling of linear electron flow from the cyclic flow by a separate, red-light driven membrane protein component. To achieve this, we apply a synthetic biology strategy to express a retinal based proton pump, proteorhodopsin, to complement the oxygenic photosynthesis in a cyanobacterium. The project comprises two steps, which is:
1.To incorporate PS3 into the thylakoid membrane and
2. To shift its absorbance maximum into the red.

We work from a well-accessible organism, the cyanobacterium Synechocystis PCC 6803, and use recombinant DNA technology to design a transgenic species, utilizing proteorhodopsin from marine bacteria. The system will be characterized by spectroscopy to analyze action spectra and proton pump capacity, with fluorescence assays for analyzing cyclic and linear electron flow, and we will use NMR and FTIR to characterize the effects on the thylakoid membrane structure.

The team that carries out this project is composed of staff members of the department of Biophysical Organic Chemistry, Gorlaeus Laboratories, Leiden University, and the Biophysics Department, Free University of Amsterdam.

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