High efficient cyclic electron flow and functional supercomplexes in Chlamydomonas cells

TL;DR

This study reveals that Chlamydomonas cells exhibit extremely high cyclic electron flow rates facilitated by a specific supercomplex formation, influenced by methyl viologen and state transition mechanisms, with implications for photosynthetic efficiency.

Contribution

It identifies a functional supercomplex responsible for high CEF rates and elucidates the role of state transitions and auxiliary proteins in its formation.

Findings

CEF rate reaches ~180-210 s-1 in presence of MV

Supercomplex formation correlates with high CEF rates

State 2 transition is required for supercomplex assembly

Abstract

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.