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Nanomachines bactériennes impliquées dans le transfert horizontal de gènes : étude structurale des protéines de l'ICESt3, un élément intégratif et conjuguatif de Streptococcus thermophilus

Abstract : Antibiotic resistance in bacteria has become part of our daily lives. These resistances are progressingfaster than new antibiotics can be found and/or synthesized : a growing problem that knows no boundaries.To acquire this resistance, bacteria exchange mobile genetic elements by horizontal transfer and by usingthe type IV secretion system (T4SS). These conjugative T4SSs are multiprotein membrane complexes, wellstudied in Gram-negative bacteria where several functional models exist. However, in Gram-positive bacteria,little information is available. The aim of this thesis was to initiate a structural work on the proteins of aT4SS ofStreptococcus thermophilus. More precisely, it is the T4SS of the integrative and conjugative elementICESt3 . On the one hand the crystallographic structure of the OrfG protein was determined and on theother hand the structure of OrfM was solved by nuclear magnetic resonance. A preliminary three-dimensionalmodel of OrfM is proposed, composed of a mixed 4-stranded sheet. The exploitation of this structure willallow the establishment of the structure-function relationships of OrfM which is supposed to act in pairwith OrfL as a helicase processivity factor for T4SS. OrfG is a protein homologous to the VirB8 proteinwhich is an assembly factor for Gram negative T4SSs. A script called MPSA_Viewer was developed in thePython language to compare OrfG to its structural homologues. This was necessary because the sequencesimilarities between these proteins were very low. All these proteins have a common fold called VirB8-likebut also disparities separating Gram-negative from Gram-positive models. These results could be correlatedto their quaternary structures. A functional dimeric assembly has been proposed in Gram-negative bacteria.The trimeric assembly of OrfG observed in the crystal and supported by the similar quaternary structuresof two other proteins of the same group suggests an original T4SS specific to Gram positive bacteria
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Submitted on : Wednesday, April 6, 2022 - 3:25:05 PM
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Julien Capelle. Nanomachines bactériennes impliquées dans le transfert horizontal de gènes : étude structurale des protéines de l'ICESt3, un élément intégratif et conjuguatif de Streptococcus thermophilus. Chimie. Université de Lorraine, 2021. Français. ⟨NNT : 2021LORR0193⟩. ⟨tel-03632862⟩

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