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Journal of Bacteriology, December 1998, p. 6625-6634, Vol. 180, No. 24
Laboratoire de Génétique
Moléculaire des Microorganismes et des Interactions Cellulaires,
CNRS UMR 5577, Institut National des Sciences Appliquées,
F-69621 Villeurbanne Cedex, France
Besides formate dehydrogenase N (FDH-N), which is involved in the
major anaerobic respiratory pathway in the presence of nitrate, Escherichia coli synthesizes a second isoenzyme, called
FDH-O, whose physiological role is to ensure rapid adaptation during a
shift from aerobiosis to anaerobiosis. FDH-O is a membrane-bound enzyme
complex composed of three subunits,
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Topological Analysis of the Aerobic Membrane-Bound
Formate Dehydrogenase of Escherichia coli
and
(FdoG),
(FdoH), and
(FdoI), which exhibit high sequence similarity to the equivalent polypeptides of FDH-N. The topology of these three subunits has been
studied by using blaM (
-lactamase) gene fusions. A
collection of 47 different randomly generated Fdo-BlaM fusions, 4 site-specific fusions, and 3 sandwich fusions were isolated along the
entire sequence of the three subunits. In contrast to previously
reported predictions from sequence analysis, our data suggested that
the 
catalytic dimer is located in the cytoplasm, with a
C-terminal anchor for
protruding into the periplasm. As expected,
the
subunit, which specifies cytochrome b, was shown to
cross the cytoplasmic membrane four times, with the N and C termini
exposed to the cytoplasm. Protease digestion studies of the
35S-labelled FDH-O heterotrimer in spheroplasts add further
support to this model. Consistently, prior studies regarding the
bioenergetic function of formate dehydrogenase provided evidence for a
mechanism in which formate is oxidized in the cytoplasm.
*
Corresponding author. Mailing address: Laboratoire de
Génétique Moléculaire des Microorganismes et des
Interactions Cellulaires, CNRS UMR 5577, Institut National des Sciences
Appliquées, Bâtiment 406, 20, avenue Albert Einstein,
F-69621 Villeurbanne Cedex, France. Phone: (33) 4 72 43 81 91. Fax:
(33) 4 72 43 87 14. E-mail: mandrand{at}insa.insa-lyon.fr.
Present address: Department of Microbiology and Molecular Genetics,
School of Medicine, Loma Linda University, Loma Linda, CA 92350.
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