Physical and biophysical studies of membrane-membrane and membrane-protein interactions require well controlled model systems. We have developed a new method to obtain a single lipid bilayer floating in excess of water, in the vicinity of a fixed bilayer. It has been prepared in the gel phase, which is not biophysically relevant. The free bilayer has been precisely characterized by Neutron and X-Ray reflectivity. By increasing the temperature up to the melting transition of the lipid chains, one can observe a spectacular change in bilayer-bilayer distance. We attribute this swelling to the bilayer's interaction of Helfrich due to the bilayer fluctuations. Theoretical implications will be discussed, allowing us to measure the bending modulus of the membrane in a large range of temperatures. In order to characterize more precisely the fluctuations of the membrane we have developed X-Ray off-specular reflectivity leading to a direct measurement of the fluctuations spectrum of the membrane. As a conclusion we have obtained a free lipid bilayer, fluid and stable, biophysically relevant substrate for bilayer-bilayer or protein-bilayer interaction studies. This allows us to characterize the fluctuations spectrum of liquid membrane in a large range of temperatures. Enantioselective reactions starting from heterochiral mixtures of precursors might have been involved during the formation of homochiral biopolymers of life at prebiotic times. Since reactions in isotropic media would lead to heterochiral products, a way to obtain oligopeptides of homochiral sequences from racemic reactants would be through the assembly of the precursor molecules into ordered architectures followed by lattice-controlled reactions. Two-dimensional (2-D) self-assemblies of amphiphilic molecules formed on water surface provide an ideal medium. Racemates of reactive α-amino acid amphiphiles were designed to self-assemble, on water surface, into 2-D crystallites of three types: racemic compounds, enantiomorphous conglomerates and enantiomerically disordered solid-solutions. Such self-assemblies could undergo polymerization to yield mixtures of oligopeptides of enantiomeric composition controlled by the packing motif of the precursor crystalline phase. The 2-D self-assembly of various racemates and the reaction induced by an appropriate catalyst were studied, directly on water surface, by grazing incidence X-ray diffraction using synchrotron radiation. The enantiomeric composition of the oligopeptides collected from the interface was determined by matrix-assisted laser-desorption-ionization time-of-flight mass spectrometry using deuterium enantio-labeled monomers. Or results show that polymerization within racemic 2-D crystallites can lead to the enhanced generation of oligopeptides with homochiral sequences through reaction between translation-related rather than glide-related molecules. When the reaction occurred primarily between glide-related molecules, heterochiral oligopeptides were obtained. The latter polymerization was taken advantage of t...

Pseudomonas entomophila is an entomopathogenic bacterium that is able to infect and kill Drosophila melanogaster upon ingestion. Its genome sequence suggests that it is a versatile soil bacterium closely related to Pseudomonas putida. The GacS/GacA two-component system plays a key role in P. entomophila pathogenicity, controlling many putative virulence factors and AprA, a secreted protease important to escape the fly immune response. P. entomophila secretes a strong diffusible hemolytic activity. Here, we showed that this activity is linked to the production of a new cyclic lipopeptide containing 14 amino acids and a 3-C 10 OH fatty acid that we called entolysin. Three nonribosomal peptide synthetases (EtlA, EtlB, EtlC) were identified as responsible for entolysin biosynthesis. Two additional components (EtlR, MacAB) are necessary for its production and secretion. The P. entomophila GacS/GacA two-component system regulates entolysin production, and we demonstrated that its functioning requires two small RNAs and two RsmA-like proteins. Finally, entolysin is required for swarming motility, as described for other lipopeptides, but it does not participate in the virulence of P. entomophila for Drosophila. While investigating the physiological role of entolysin, we also uncovered new phenotypes associated with P. entomophila, including strong biocontrol abilities.