line decor
     [Accueil]
line decor

Presentation of the laboratory

Research Domains

Laboratory Publications

Scientific Instrumentation

University Relations

Jobs

People/Oganization chart


Links :

ChlamyStation : Chlamydomonas Photosynthetic Mutant Collection

PredAlgo : multi-subcellular localization prediction tool dedicated to Algae

IBPC : Institut de Biologie Physico-Chimique

LabEx DYNAMO

Function and building of the photosynthetic apparatus : from light to life.

   Photosynthesis, as the only source of renewable energy in the biosphere, has first allowed the development of life on earth and now plays a crucial role in maintaining the geochemical and biological equilibria of our planet. It is driven by plants, microalgae or cyanobacteria which utilize light as a fuel to sustain their growth. Our laboratory studies the building and the function of the photosynthetic apparatus in vegetables.

   The conversion of light energy into useful chemical energy is performed at the level of intracellular membranes which embed a variety of proteins and cofactors delicately assembled within supramolecular structures. Both the absorption of light energy and its conversion into chemical energy through a sequence of electron transfer reactions are catalyzed by these multimolecular complexes. This sequence of oxydo-reduction reactions leads to the evolution of dioxygen and the production of strong reductants as well as ATP. These compounds allow carbone dioxyde assimilation into carbone hydrates which constitute the ubiquitous fuel at the cellular level. To study the photosynthetic apparatus, our laboratory makes mostly use -although not exclusively- of a unicellular photosynthetic organism, the green alga Chlamydomonas reindhardtii, which allows a genetic approach that proves essential when aiming at introducing perturbations of the photosynthetic processes in a controlled manner. Our laboratory has built up a unique library of Chlamydomonas photosynthetic mutants (ChlamyStation). We have developed as well original spectroscopic devices to study the photosynthetic function in vivo, in leaves or intact cells.

 

french version
    Screening of mutants altered in their photosynthetic function based on the time-resolved analysis of the changes of the fluorescence yield.
The top part of the figure illustrates the fluorescence changes, as a function of the duration of the illumination time, from a control plantlet (in red) and a mutant lacking the cytochrome b6f complex (in black). The bottom part of the figure illustrates a similar strategy aiming at identifying, among various Chlamydomonas reinhardtii colonies, those strains characterized by altered kinetics of fluorescence changes (A) and thus affected, to different extents, in their photosynthetic function.
   

 

 

The function of the photosynthetic apparatus.

   In the photosynthetic membranes one can find a limited number of proteins involved in the conversion of light into chemical energy. Their functions may be studied either after biochemical purification or in situ, in membrane fragments, or even in vivo under physiological conditions. Our laboratory has put efforts in studying the photosynthetic function at these different levels of integration, with, however, more emphasis on the in vivo studies for only they allow addressing the question of the metabolic regulation of photosynthesis and consequently its physiology. This approach relies on experimental set-ups which meet the requirements for detecting functional signals of cofactors present at under-optimal concentrations and moreover in extremely complex environment. The ability to trigger the primary reactions by light pulses of extremely short duration is unique in biology and allows the study of a sequence of reactions the time domain of which ranges from the femtosecond to several minutes. Electron transfer reactions within a protein or between two distinct proteins may thus be studied under various conditions relevant to the physiological regulation processes of the photosynthetic function. As an example, this approach has led to the characterization of two different operating modes for the photosynthetic electron transfer chain: one allowing carbone dioxyde fixation, the other one being exclusively devoted to ATP production, a molecule involved in the catalysis of numerous endergonic reactions in biological tissues. In addition, it allows studying the diversity of the strategies followed by a variety of photosynthetic organisms to cope with the physiological fluctuations they are submitted to by their environment. Thereby, we aim at widening the scope of the undergoing projects which ranges from the structure/function relationship at the atomic or molecular level to the ' functional ecology'.

 

How is the photosynthetic apparatus assembled ?

   The proteins involved in the photosynthetic function are multimolecular assemblies comprising several polypeptide chains and several molecules such as pigments -chlorophylls and carotenoids- or redox cofactors participating to the electron chain such as hemes or iron sulfur clusters. The assembly of these oligomeric protein in the chloroplast membranes requires two distinct genomes, the nuclear and chloroplast genomes. Each one only encodes a fraction of the sub-units the assembly of which constitutes the proteins complexes of the photosynthetic chain. The ancestor of the chloroplast, the organelle where photosynthesis occurs, is an endosymbiotic cyanobacterium. Although the genetic machinery involved in protein synthesis is thus of procaryotic origin, the chloroplastic gene expression processes are original for they require the involvement of nuclear encoded proteic factors which rule the post-transcriptional steps of chloroplastic genes expression. Our laboratory studies the relationship between transcription and translation in chloroplast, with an emphasis on the characterization of the mechanisms by which these nuclear encoded factors participate to the chloroplast genome expression. The assembly of the different nuclear encoded sub-units with their chloroplastic partners in a single functional complex is an issue under careful scrutiny in the laboratory. The stoichiometric accumulation, in the chloroplast, of the different sub-units within a single protein is made possible by regulatory mechanisms at the translation or proteolysis levels.

 
   

Both the absoption of light energy and its conversion into chemical energy through a series of electron transfer reactions are catalysed by a set of multimolecular complexes.

   
© UMR7141