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 b6/f complex and the ATPase. The two photosystems
operate in series during linear electron flow to split H2O 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 C3-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 P700 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 b6/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 b6/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 and PGR5 in the
switch from linear to cyclic electron flow depending on the redox
state of the chloroplast.