PETO Interacts with Other Effectors of Cyclic Electron Flow in Chlamydomonas.

While photosynthetic linear electron flow produces both ATP and NADPH， cyclic electron flow (CEF) around photosystem I (PSI) and cytochrome b6f generates only ATP. CEF is thus essential to balance the supply of ATP and NADPH for carbon fixation; however， it remains unclear how the system tunes the relative levels of linear and cyclic flow. Here， we show that PETO， a transmembrane thylakoid phosphoprotein specific of green algae， contributes to the stimulation of CEF when cells are placed in anoxia. In oxic conditions， PETO co-fractionates with other thylakoid proteins involved in CEF (ANR1， PGRL1， FNR). In PETO-knockdown strains， interactions between these CEF proteins are affected. Anoxia triggers a reorganization of the membrane， so that a subpopulation of PSI and cytochrome b6f now co-fractionates with the CEF effectors in sucrose gradients. The absence of PETO impairs this reorganization. Affinity purification identifies ANR1 as a major interactant of PETO. ANR1 contains two ANR domains， which are also found in the N-terminal region of NdhS， the ferredoxin-binding subunit of the plant ferredoxin-plastoquinone oxidoreductase (NDH). We propose that the ANR domain was co-opted by two unrelated CEF systems (PGR and NDH)， possibly as a sensor of the redox state of the membrane.