  A very high rate for cyclic electron flow (CEF) around PSI (~180 s-1 or 210
s-1 in minimum medium or in the presence of a carbon source respectively) is
measured in the presence of methyl viologen (MV) in intact cells of
Chlamydomonas reinhardtii under anaerobic conditions. The observation of an
efficient CEF in the presence of methyl viologen is in agreement with the
previous results reports of Asada et al in broken chloroplasts (Plant Cell
Physiol. 31(4) (1990) 557-564). From the analysis of the P700 and PC absorbance
changes, we propose that a confinement between 2 PC molecules, 1 PSI and 1
cytb6f corresponding to a functional supercomplex is responsible for these high
rates of CEF. Supercomplex formation is also observed in the absence of methyl
viologen, but with lower maximal CEF rate (about 100 s-1) suggesting that this
compound facilitates the mediation of electron transfer from PSI acceptors to
the stromal side of cytb6f. Further analysis of CEF in mutants of Chlamydomonas
defective in state transitions shows the requirement of a kinase-driven
transition to state 2 to establish this functional supercomplex configuration.
However, a movement of the LHCII antennae is not involved in this process. We
discuss the possible involvement of auxiliary proteins, among which is a small
cytb6f-associated polypeptide, the PETO protein, which is one of the targets of
the STT7 kinase.
