Characterization of the Novel Photosynthetic Protein PPP7 involved in Cyclic Electron Flow around PSI

Photosynthetic organisms are able to convert light energy into chemical energy by the operation of the two photosystems, the cytochrome b 6 /f complex and the ATPase. The two photosystems operate in series during linear electron flow to split H 2 O and to generate NADP +. During electron transport, a pH gradient is generated across the thylakoid membrane which is used for the generation of ATP. In addition to the linear electron transport mode, ATP can also be produced via cyclic electron flow around photosystem I (CEF). The physiological role of CEF in vascular plants with C 3-type photosynthesis is still not solved. Potential functions of CEF are (i) the dissipation of excessive light energy by increasing non-photochemical quenching (NPQ); (ii) ATP synthesis during steady-state photosynthesis; (iii) the regulation of the stromal oxidation state under stress conditions and under conditions when the Calvin cycle is not available as a sink for NADPH. With exception of the thylakoid NADPH-dehydrogenase complex and the stromal protein PGR5, the components that contribute to CEF are still unknown. Obscure is also the regulation that controls the switch from linear to cyclic flow. We have identified a novel transmembrane protein, named PPP7, which is located in thylakoids of photoautotrophic eukaryotes. Mutants lacking PPP7 exhibit the same phenotype as plants missing PGR5. These mutants show reduced NPQ, decreased P 700 oxidation and perturbation of ferredoxin-dependent CEF. The work described in this thesis demonstrates that PPP7 and PGR5 interact physically, and that both co-purify with photosystem I. PPP7 does also interact in yeast assays with the cytochrome b 6 /f complex, as well as with the stromal proteins ferredoxin (Fd) and ferredoxin-NADPH oxido-reductase (FNR), but PPP7 is not a constitutive component of any of the major photosynthetic complexes. In consequence, the existence of a PPP7/PGR5 complex integrated in the thylakoid membrane and facilitating CEF around PSI in eukaryotes, possibly by shuttling electrons together with ferredoxin and the FNR from photosystem I to the cytochrome b 6 /f complex, is proposed. Moreover, CEF is enhanced in the Arabidopsis psad1 and psae1 mutants with a defect in photosystem I oxidation in contrast to the cyanobacterial psae mutant which exhibits an decreased CEF, pointing to fundamental mechanistic differences in the cyclic electron flow of cyanobacteria and vascular plants. The Arabidopsis psad1 and psae1 mutants also show higher contents of ferredoxin and of the PPP7/PGR5 complex, supporting a role of PPP7 …