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Accueil > Recherche > Equipes de recherche > Génétique microbienne > Présentation / résumé

Génétique microbienne

Présentation

We have an interest in understanding the pathways of storage polysaccharide synthesis and mobilization in general with a special emphasis on the study of alpha-glucans storage compounds such as starch and glycogen.

Glycogen metabolism in fungi and mammalian cells has received a lot of attention and has been thoroughly studied. Comparatively, starch metabolism in plants, glycogen metabolism in other non fungal lower eukaryotes and glycogen metabolism in bacteria and archaea have all received much less attention and are rather poorly understood.

Our initial research focused on working out the major aspects of starch metabolism through the study of mutants affected at various steps of the pathway. In the late eighties and early nineties we chose the unicellular green alga Chlamydomonas as our major model for this purpose. Indeed Chlamydomonas defined at the time, the ideal system for the fast isolation and study of mutants.

Ten loci affecting starch synthesis were thoroughly studied throughout the years. Our group made essential contributions to the understanding of starch metabolism that are familiar to most of the specialists in this field. These contributions can be summarized as understanding the major steps responsible for distinguishing starch from glycogen synthesis, working out the detailed mechanism of amylose synthesis, contributing to the understanding of the precise function of the multiple forms of enzymes responsible for amylopectin synthesis.

At the end of this decade of research it appeared to us that functional approaches in Chlamydomonas would be soon out-competed by reverse genetic approaches in Arabidopsis. Indeed the Arabidopsis genome sequence was obtained first together with a complete collection of tagged mutants for nearly all loci of the plant genome. We therefore engaged around 1999-2000 in large scale research programs involving Arabidopsis functional genomics. Christophe d’Hulst, a former member of the microbial genetics research group, took these aspects in charge and now is independently continuing in this very productive direction as a head of the plant glycobiology group together with Fabrice Wattebled.

However, our group is now far more attracted by another biological question. The latter consisted of the evolutionary path followed by the starch metabolism network of genes. Indeed evo-devo type of approaches have stimulated research in a lot of different areas of developmental biology. We believe the same kind of approaches and reasoning could be applied fruitfully to biochemical pathways. However true evo-devo approaches are set within a time scale of events dating from a few million years up to at most a few hundred million of years. Moreover the phylogenetic origin of the species studied for that purpose are usually perfectly understood. The problem with important and widespread biochemical pathways is that these are comparatively much older dating back to hundreds of millions and often billion of years. In addition the background of events that have shaped them is nearly completely unknown. Although numerous fascinating theories can be put forward concerning the origin of eukaryotes archaea or bacteria, there is no present consensus on these questions. With such unknowns retracing the evolutionary steps that have generated the major biochemical pathways can be a highly difficult and speculative exercise.

As to the origin of the major plant specific pathways the situation is fortunately much clearer. Indeed there is now a very large consensus concerning the origin of plants and their monophyletic origin has been accepted. Plants (that is to say red algae, green algae and land plants and glaucophytes) originate from a single endosymbiotic event involving a cyanobacterial symbiont and a heterotrophic eukaryote host. While some plant pathways are still either completely cyanobacterial or eukaryotic in their essence (for instance photosynthesis or mitosis) others result from the merging of pre-existing pathways while some others result from subsequent acquisition and evolution.

Starch is at first glance an old plant specific pathway. Indeed this polysaccharide is only found in plants or their endosymbiotic derivatives (such as cryptophytes, dinoflagellates and apicomplexan parasites). Our recent goal has been to understand how the storage polysaccharide pathways of the cyanobacterial symbiont and its eukaryote host have merged to generate starch.

This involves studying in detail through a combination of biochemical and genetic approaches a number species located at phylogenetically relevant positions within the tree of life including cyanobacteria, dinoflagellates, glaucophytes, rhodophytes, cryptophytes. A major distinction in our novel approaches is that we don’t rely only on bioinformatics study of completed genome sequences but rather on functional mutant studies aimed at understanding how a similar enzyme has evolved and changed to fulfill different functions in the same polysaccharide metabolism enzyme network. A good example of that is given on our recent approaches comparing the phenotype of debranching enzyme or phosphorylase defective mutants in bacteria and algae. By working-out these aspects we recently stumbled, quite by chance, on what we feel explains the major aspects of plastid endosymbiosis and the birth of the plant kingdom. This contribution is now published known as the “ménage à trois” hypothesis. Our present working hypothesis is that the symbiotic flux of carbon at the onset of plastid endosymbiosis involved glycogen metabolism through the coding by 3 genomes of enzymes required to establish the flux. These genomes consisted of the incipient cyanobacterium (the cyanobiont), a helper chlamydial symbiont or pathogen and the eukaryotic host. This proposal has major consequences on our view of organelle biogenesis and emphasizes the importance of phagocytosis and energy parasites (rickettsiae and chlamydiae) as major drivers of eukaryote evolution.