Awardees

Abstract:

About 1.8 Ga ago, cyanobacteria were phagocyted by eukaryotes, leading to primary endosymbiosis and giving rise to chloroplast. Since then, other eukaryotes have included photosynthetic eukaryotes (secondary/tertiary endosymbiosis), which appear to have evolved many times independently. Despite the ubiquity of photo-endosymbiosis in the environment, these early mechanisms remain widely unknown. The aim is to study the first steps leading to the maintenance of a photosynthetic organism within a eukaryote. To this end, we are using unicellular models, the host ciliate Tetrahymena thermophila and the photosynthetic alga Chlorella variabilis and the cyanobacterium Synechococcus elongatus. These models allowed us to study the selective advantage that phagocytosis of a photosynthetic organism can bring to its host under unfavorable conditions. The results show the importance of _O2 for a better survival of T. thermophila having phagocytosed photosynthetic organisms in anaerobic conditions.

Key words:

Photosymbiosis, evolution, oxygen, phagocytosis.

Summary:

Microorganisms live in complex communities, perceiving and responding to their environment by exchanging signals and resources. This complexity and interconnectedness endow the entire community with new functions and enhanced performance that cannot be predicted simply by analyzing the genomes of the species present. Synthetic consortia, inspired by natural ecosystems, offer a valuable model for exploring, deciphering and controlling for potential applications inter-species interactions under simplified conditions. This work focuses on a model consortium associating Nitratidesulfovibrio vulgaris Hildenborough(NvH) and Clostridium acetobutylicum. Within this system, under conditions of nutritional stress, metabolic and physical interactions are set in motion, leading to higher bio-hydrogen production than in pure culture. While the presence of AI-2 quorum-sensing molecules has been demonstrated to be essential for these interactions, the aim of this thesis was to decipher the molecular basis of this interaction. An in silico approach was defined and implemented to identify potential AI-2 QS players in C. acetobutylicum and NvH. The results suggested the existence of previously uncharacterized, non-canonical Lsr-type QS pathways in these bacteria. In vitro approaches demonstrated the function of LsrK-like and LsrR-like proteins in NvH, and thus their role in AI-2-type signaling. LsrK-like is a kinase with the dual function of phosphorylating AI-2 and glycerol, while LsrR-like is a DNA-binding regulator that responds exclusively to AI-2 phosphorylation under the conditions tested. These results reflect an adaptation of the NvHbacterium to perceive an environmental signal through remodeling of the glycerol signaling pathway. Indeed, this hypothesis is supported by a transcriptomic study which identified the NvHgenes regulated by AI-2. This study identified a transporter that may be involved in AI-2 import, thus completing the QS signalling pathway, as well as a metabolic remodelling of NvHbacteria in the presence of AI-2, such as activation of the thiamine synthesis pathway. In parallel, a meta-transcriptomic analysis was used to study the genes regulated within the consortium, pointing to metabolic pathways significantly impacted by the interaction between the two bacteria. The results show modifications in the production of certain amino acids and in other quorum sensing pathways in C. acetobutylicum. This work necessitated the development of a new absolute quantification method using digital droplet PCR to assess the cell ratio within a synthetic consortium. This work highlights the importance of intra- and interspecific communication systems in regulating interactions within the consortium, and opens up avenues for future studies on the biocontrol of microbial systems.

Co-supervisors :

• Marie-Thérèse Giudici-Orticoni (BIP)
• Christophe Bordi (LISM)

Abstract:

Bacteria produce small molecules known as secondary metabolites, which interest research for their numerous therapeutic properties and their role within the biotope. Myxobacteria are Gram-negative bacteria described as multi-producers of secondary metabolites. However, their identification and production in the laboratory is complex. During my thesis, I used the myxobacterium, M. xanthus, to develop a 2PRIM-BOOST approach, which combines optimization of the M. xanthus culture protocol and synthetic biology. The advantages of this approach are an accumulation of compounds involved in the biosynthesis of secondary metabolites, and a simplified metabolome facilitating structural identification and the demonstration of biological activities. As a proof of concept, the combination of the different tools enabled us to significantly increase myxoprincomide production. Once validated, this method enabled me to discover a new metabolite, and analogues, which remain to be characterized. For their biosynthesis, some metabolites require proteins with an Fe-S cofactor. I have studied the process of Fe-S protein biogenesis in M. xanthus and discovered a genetic context in which the expression of Fe-S cluster biogenesis machineries is maximal. All my work will be used to identify unknown secondary metabolites of M. xanthus, and more broadly those of myxobacteria, whether or not they depend on Fe-S proteins.

Co-supervisors :

• Corinne AUBERT (LCB)
• Marc MARESCA (iSm2)
• Béatrice PY (LCB)