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The succinate:menaquinone reductase of Bacillus cereus - characterization of the membrane-bound and purified enzyme

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Abstract:

Utilization of external succinate by Bacillus cereus and the properties of the purified succinate:menaquinone-7 reductase (SQR) were studied. Bacillus cereus cells showed a poor ability for the uptake of and respiratory utilization of exogenous succinate, thus suggesting that B. cereus lacks a specific succinate uptake system. Indeed, the genes coding for a succinate-fumarate transport system were missing from the genome database of B. cereus. Kinetic studies of membranes indicated that the reduction of menaquinone-7 is the rate-limiting step in succinate respiration. In accordance with its molecular characteristics, the purified SQR of B. cereus belongs to the type-B group of SQR enzymes, consisting of a 65-kDa flavoprotein (SdhA), a 29-kDa iron-sulphur protein (SdhB), and a 19-kDa subunit containing 2 b-type cytochromes (SdhC). In agreement with this, we could identify the 4 conserved histidines in the SdhC subunit predicted by the B. cereus genome database. Succinate reduced half of the cytochrome b content. Redox titrations of SQR-cytochrome b-557 detected 2 components with apparent midpoint potential values at pH 7.6 of 79 and -68 mV, respectively; the components were not spectrally distinguishable by their maximal absorption bands as those of Bacillus subtilis. The physiological properties and genome database analyses of B. cereus are consistent with the cereus group ancestor being an opportunistic pathogen.

L’utilisation de succinate exogène par Bacillus cereus et les propriétés de la succinate:menaquinone-7 (MK7) réductace (SQR) purifiée ont été étudiées. Les cellules de B. cereus démontrent une faible capacité de captation et d’utilisation respiratoire de succinate exogène, ce qui suggère qu’un système de captation spécifique de succinate soit absent chez B. cereus. En effet, les gènes codant le système de transport succinate:fumarate sont absents des bases de données génomiques de B. cereus. Des études cinétiques faites à partir de membranes ont indiqué que la réduction de MK7 constituait l’étape limitante de la respiration à partir du succinate. Conformément à ses caractéristiques moléculaires, la SQR purifiée de B. cereus appartient au groupe des enzymes SQR de type B, consistant en une flavoprotéine de 65 kDa (SdhA), une protéine à fer-soufre de 29 kDa (SdhB) et une sous-unité de 19 kDa contenant 2 cytochromes de type-b (SdhC). Conformément à ce modèle, nous avons pu identifier les 4 histidines conservées la sous-unité SdhC prédites par les bases de données génomiques de B. cereus. Le succinate réduisait la moitié de contenu en cytochrome b. La titration redox de la SQR-cytochrome b-557 a détecté 2 composantes ayant des valeurs apparentes de potentiel central (Em,7.6) de 79 et -68 mV respectivement qui ne pouvaient se distinguer de façon spectrale par leurs bandes d’absorption maximales, comparativement à leurs contreparties chez B.acillus subtilis. Les analyses des propriétés physiologiques et des bases de données génomiques de B. cereus concordent avec le fait que l’ancêtre du groupe cereus soit un pathogène opportuniste.

Document Type: Research Article

Publication date: 2008-06-01

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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