Photosynthetic electron transfer through the cytochrome b6f complex can bypass cytochrome f.

The cytochrome b(6)f complex is an obligatory electron transfer and proton-translocating enzyme in all oxygenic photosynthesis. Its operation has been described by the "Q-cycle." This model proposes that electrons are transferred from plastoquinol to plastocyanin (the reductant of P700 in Photosystem I) through, obligatorily in series, the iron-sulfur and the cytochrome f redox centers in the cytochrome b(6)f complex. However, here we demonstrate that (a) the iron-sulfur center-dependent reductions of plastocyanin and P700 are much faster than cytochrome f reduction, both in Chlamydomonas reinhardtii cytochrome f mutants and in the wild type, and (b) the steady-state photosynthetic electron transport does not correlate with strongly inhibited cytochrome f reduction kinetics in the mutants. Thus, cytochrome f is not an obligatory intermediate for electrons flowing through the cytochrome b(6)f complex. The oxidation equivalents from Photosystem I are delivered to the high potential chain of the cytochrome b(6)f complex both at the cytochrome f level and, independently, at another site connected to the quinol-oxidizing site, possibly the iron-sulfur center.