Jérôme Lavergne

Redox and ATP control of photosynthetic cyclic electron flow in Chlamydomonas reinhardtii (I) aerobic conditions

Assimilation of atmospheric CO2 by photosynthetic organisms such as plants, cyanobacteria and green algae, requires the production of ATP and NADPH in a ratio of 3:2. The oxygenic photosynthetic chain can function following two different modes: the linear electron flow which produces reducing power and ATP, and the cyclic electron flow which only produces ATP. Some regulation between the linear and cyclic flows is required for adjusting the stoichiometric production of high-energy bonds and reducing power. Here we explore, in the green alga Chlamydomonas reinhardtii, the onset of the cyclic electron flow during a continuous illumination under aerobic conditions. In mutants devoid of Rubisco or ATPase, where the reducing power cannot be used for carbon fixation, we observed a stimulation of the cyclic electron flow. The present data show that the cyclic electron flow can operate under aerobic conditions and support a simple competition model where the excess reducing power is recycled to match the demand for ATP.

Modification of the pheophytin midpoint potential in photosystem II: Modulation of the quantum yield of charge separation and of charge recombination pathways

We investigated the dependence of both the quantum yield of charge separation and pathways of charge recombination on the free energy level of the radical pair state P+Ph− in photosystem II. This was done by comparing the basal (F0) fluorescence yield and the recombination rate of the S2QA− state in various strains of Chlamydomonas reindhardtii in which the strength of a H-bond to the pheophytin bound to the D1 subunit has been modified by site-directed mutagenesis. In agreement with previous results with homologous mutants in the cyanobacterium Synechocystis, the quantum yield decreased and the recombination of S2QA− was slowed down when the energy level of the radical pair was increased. The decreased quantum yield is analyzed in terms of a modified equilibrium between exciton and radical pair. The effects on the recombination rate confirm that in the wild type the process involves the P+Ph− state. Analysis of these results shows that, as in the case of Synechocystis, the energy level of the P+Ph− is less negative than currently thought by about 250 meV. An important consequence is a similar upward revision of the potential of the P+/P couple.

Functional Coupling Between Reaction Centers and Cytochrome bc 1 Complexes

Light-induced cyclic electron transfer in purple bacteria involves two integral membrane protein complexes, the reaction center (RC) and the cytochrome bc 1 complex, and two mobile carriers that shuttle between them. The mobile carrier on the acceptor side of the RC is a quinone molecule, confined to the hydrophobic membrane/protein regions. More diversity is found for the donor side, starting with the RC, which may or may not possess a multiheme donor subunit. Depending on species, the multiheme subunit includes three or four c-type hemes with alternating high and low midpoint potentials so that the electron transfer involves a succession of uphill and downhill steps. The donor side mobile carrier, confined to the periplasmic space, is either a soluble cytochrome (c 2 or c 8), a membrane-anchored c-type cytochrome, or a high-potential iron-sulfur protein (HiPIP). The stoichiometric ratios between the components of the photosynthetic chain (generally in the order: quinones > RC > mobile cytochrome > bc 1) are variable depending on species and growth conditions. In many species where the core antenna ring surrounds the RC completely, the quinone circuit requires the crossing of this barrier. For species endowed with a low bc 1:RC ratio, the diffusion of the mobile carriers over relatively long ranges is mandatory for connecting the distant partners. In some Rhodobacter species, with a high bc 1:RC ratio, evidence has been found for the formation of specific supercomplexes associating all the components of the cyclic transfer in largely independent functional units. The mechanisms that may be responsible for the confinement of the mobile carriers in such systems are discussed.

Partial equilibration of photosynthetic electron carriers under weak illumination: a theoretical and experimental study

When submitting photosynthetic material to weak illumination, in uncoupled conditions, quasi-equilibrium states of the redox carriers are attained that should presumably reflect the equilibrium constants deduced from the mid-point potentials. However, the apparent constants experimentally obtained are, in several instances, much lower than expected. In order to account for such discrepancies, we examine the consequences of ‘clustering’ of neighboring carriers, assuming rapid equilibration within a cluster, but much slower relaxation towards the intercluster equilibrium. A general treatment is developed and its predictive content is discussed. Its application to two experimental systems is described, namely the donor chain of isolated reaction centers of Rhodopseudomonas viridis, and the donor chain of Rhodobacter sphaeroides in vivo, investigated in the accompanying paper (Joliot, P., Vermeglio, A. and A. Joliot) (1989) Biochim. Biophys. Acta 975, 336–346). Possible extension of this class of hypothesis to the donor chain of Photosystem I and to the acceptor chainof Photosystem II in chloroplasts is suggested.

Organisation and function of the Phaeospirillum molischianum photosynthetic apparatus

We have investigated the organisation of the photosynthetic apparatus in Phaeospirillum molischianum, using biochemical fractionation and functional kinetic measurements. We show that only a fraction of the ATP-synthase is present in the membrane regions which contain most of the photosynthetic apparatus and that, despite its complicated stacked structure, the intracytoplasmic membrane delimits a single connected space. We find that the diffusion time required for a quinol released by the reaction centre to reach a cytochrome bc1 complex is about 260 ms. On the other hand, the reduction of the cytochrome c chain by the cytochrome bc1 complex in the presence of a reduced quinone pool occurs with a time constant of about 5 ms. The overall turnover time of the cyclic electron transfer is about 25 ms in vivo under steady-state illumination. The sluggishness of the quinone shuttle appears to be compensated, at least in part, by the size of the quinone pool. Together, our results show that P. molischianum contains a photosynthetic system, with a very different organisation from that found in Rhodobacter sphaeroides, in which quinone/quinol diffusion between the RC and the cytochrome bc1 is likely to be the rate-limiting factor for cyclic electron transfer.