PGR5 is required for efficient Q cycle in the cytochrome b6f complex during cyclic electron flow.

Proton gradient regulation 5 (PGR5) is involved in the control of photosynthetic electron transfer, but its mechanistic role is not yet clear. Several models have been proposed to explain phenotypes such as a diminished steady-state proton motive force (pmf) and increased photodamage of photosystem I (PSI). Playing a regulatory role in cyclic electron flow (CEF) around PSI, PGR5 contributes indirectly to PSI protection by enhancing photosynthetic control, which is a pH-dependent down-regulation of electron transfer at the cytochrome b(6)f complex (b(6)f). Here, we re-evaluated the role of PGR5 in the green alga Chlamydomonas reinhardtii and conclude that pgr5 possesses a dysfunctional b(6)f. Our data indicate that the b(6)f low-potential chain redox activity likely operated in two distinct modes - via the canonical Q cycle during linear electron flow and via an alternative Q cycle during CEF, which allowed efficient oxidation of the low-potential chain in the WT b(6)f. A switch between the two Q cycle modes was dependent on PGR5 and relied on unknown stromal electron carrier(s), which were a general requirement for b(6)f activity. In CEF-favoring conditions, the electron transfer bottleneck in pgr5 was the b(6)f, in which insufficient low-potential chain redox tuning might account for the mutant pmf phenotype. By attributing a ferredoxin-plastoquinone reductase activity to the b(6)f and investigating a PGR5 cysteine mutant, a current model of CEF is challenged.