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Défense de Streptococcus thermophilus contre le stress oxydatif : existence d'un système de réponse, construction et sélection d'une collection de mutants, identification de gènes impliqués

Abstract : Streptococcus thermophilus is an anaerobic aerotolerant organism. A hydrogen peroxide defence system had been shown to be inducible in the CNRZ368 strain, and may take part in its aerotolerance . In order to identify genes involved in S. thermophilus defence against oxidative stress, clones from a mutant collection obtained by insertional mutagenesis, had been selected for their sensitivity or resistance to an oxidative stress. Molecular analysis of mutants selected under these conditions showed that their disruption sites can be classified in 4 categories: those that carried genes potentially linked to oxidative stress defence, those potentially involved in cell wall biosynthesis, those potentially involved in general metabolism, those that carried genes of unknown function. Two mutants, disrupted in genes homologous to RodA et PBP2b were affected in their cell shape, confirming that these genes were probably functional homologues of RodA and PBP2b genes involved in elongation of lateral cell wall. Two other mutants were affected into two distinct ORF but localised in the same locus. This region contained at least 4 ORF whose hypothetical translation products are homologous to proteins involved in Fe-S centers maturation. A transcriptional analysis showed their cotranscription. Directed mutagenesis affecting one of the 4 ORF (nifU-like) allowed to demonstrate its direct involvement in oxidative stress defence.Moreover, study of CNRZ368 colonial phenotype revealed the existence of spontaneous variants that appeared with a frequency of almost one percent. Detection of variant phenotypes was conditional to oxygen presence during growth. In addition, variant clones often displayed a better resistance to oxidative stress than did the wild-type strain, suggesting a link (although it is not a strict link) between these variants and the resistance against oxidative stress level.
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http://docnum.univ-lorraine.fr/prive/SCD_T_2002_0281_THIBESSARD.pdf
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Submitted on : Thursday, March 29, 2018 - 10:45:06 AM
Last modification on : Wednesday, October 14, 2020 - 3:53:31 AM

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  • HAL Id : tel-01746801, version 1

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Annabelle Thibessard. Défense de Streptococcus thermophilus contre le stress oxydatif : existence d'un système de réponse, construction et sélection d'une collection de mutants, identification de gènes impliqués. Génétique. Université Henri Poincaré - Nancy 1, 2002. Français. ⟨NNT : 2002NAN10281⟩. ⟨tel-01746801⟩

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