Complex I of Rhodobacter capsulatus and its role in reverted electron transport
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© Springer-Verlag 1998
O R I G I N A L PA P E R
Stefan Michael Herter · Christiane Maria Kortlüke · Gerhart Drews
Complex I of Rhodobacter capsulatus and its role in reverted electron transport
Received: 28 February 1997 / Accepted: 28 August 1997
Abstract The activities of NAD+-photoreduction and NADH/decyl-ubiquinone reductase in membrane preparations of Rhodobacter capsulatus changed to the same extent under different conditions. These results indicated that NADH:ubiquinone oxidoreductase (complex I) catalyzes the electron transport in the downhill direction (respiratory chain) and in the uphill direction (reverted electron flow). This conclusion was confirmed by the characterization of a complex-I-deficient mutant of R. capsulatus. The mutant was not able to reduce NAD+ in the light. Since this mutant was not able to grow photoautotrophically, we concluded that complex I is the enzyme that catalyzes the reverted electron flow to NAD+ to provide reduction equivalents for CO2 fixation. Complex I is not essential for the reverted electron flow to nitrogenase since the mutant grew under nitrogen-fixing conditions. As shown by immunological means, NuoE, a subunit of complex I from R. capsulatus having an extended C-terminus, was modified depending on the nitrogen source present in the growth medium. When the organism used N2 instead of NH4+, a smaller NuoE polypeptide was synthesized. The complex-I-deficient mutant was not able to modify NuoE. The function of the modification is discussed. Key words Rhodobacter capsulatus · Complex I · Reverted electron flow · NAD+-photoreduction · CO2 fixation · N2 fixation · NuoE · NuoF Abbreviations Ap Ampicillin · Cm Chloramphenicol, complex I NADH:ubiquinone oxidoreductase · D Dilution rate · Decyl-ubiquinone 2,3-dimethoxy5-methyl-6-decyl-1,4-benzoquinone · Gm Gentamicin · Km Kanamycin
S. M. Herter · C. M. Kortlüke · G. Drews (Y) Institut für Biologie 2, Mikrobiologie, Albert-Ludwigs-Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany Tel. +49 761 203 2607; Fax +49 761 203 2779 e-mail [email protected]
Introduction Rhodobacter capsulatus generates a proton motive force either by a light-driven electron transport system under anoxic conditions or by a respiratory chain-coupled mechanism. It is known as one of the metabolically most versatile bacteria [for a review, see Drews and Imhoff (1991)]. The respiratory chain is related to that in mitochondria but is branched (Baccarini-Melandri et al. 1973; Hüdig and Drews 1984; Gabellini 1988; Sled’ et al. 1993). Under anoxic photoautotrophic growth conditions, NAD+ cannot be reduced by direct electron transport from the reaction center as in oxygenic photosynthesis (Pierson and Olson 1987). Photoautotrophic growth with CO2 as the only carbon source and H2 as the electron donor requires an enzymatic pathway to provide reduction equivalents in the form of NADH. Formation of H+ and electrons from H2 is catalyzed by the membrane-bound uptake hydrogenase. Electrons are possibly fed into a reverse uphill electron
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