Engineering retinal-based phototrophy via a complementary photosystem in Synechocystis sp. PCC6803

To achieve success in setting up sustainability applications, photosynthetic organisms are required to convert solar energy with the highest efficiency. A widely proposed method to do this is to expand their effective absorption spectrum into the infrared (i.e. beyond 700 nm). Introducing an infrared-absorbing, retinal-based proton pump, as a substitute for PSI, is a promising approach to achieve this. This thesis focuses on expressing two such retinal-based pumps, proteorhodopsin (PR) and Gloeobacter rhodopsin (GR), into Synechocystis sp. PCC 6803 and its PSI-deletion derivate. In these strains we explored the stimulatory effect of these pumps on the growth of their host. To facilitate expression of PR and GR variants with a modified chromophore, we also investigated retinal metabolism in Synechocystis.
Our results show that, upon use of the psbA2 promoter, bacterial rhodopsins (PR or GR) are expressed in Synechocystis, at a level of up to 10⁵ molecules per cell, presumably as pentamers and trimers, respectively, with approximately equal distribution (on a protein-content basis) over the thylakoid and the cytoplasmic membrane fraction. Functional assays have revealed that PR but not GR can enhance the growth rate of Synechocystis and its ΔPSI derivative strain. Notably, the stimulatory effect is most significant in the ΔPSI strain. Moreover, our results confirm that Synechocystis sp. PCC 6803 has the capacity to synthesize all-trans-retinal, thereby making use of the SynACO enzyme (encoded by sll1541). By supplying a red-shifted retinal analogue to a sll1541-knockout mutant we could show that an infrared-absorbing PR derivative is formed in Synechocystis.