Paracoccus denitrificans is able to grow on the C
1 compounds methanol and methylamine. These compounds are oxidized to formaldehyde which is subsequently oxidized via formate to carbon dioxide. Biomass is produced by carbon dioxide fixation via the ribulose biphosphate pathway. The first oxidation reaction is catalyzed by the enzymes methanol dehydrogenase and methylamine dehydrogenase, respectively. Both enzymes contain two different subunits in an
22 configuration. The genes encoding the subunits of methanol dehydrogenase (
moxF and
moxI) have been isolated and sequenced. They are located in one operon together with two other genes (
moxJ and
moxG) in the gene order
moxFJGI. The function of the
moxJ gene product is not yet known.
MoxG codes for a cytochrome
c
551i
, which functions as the electron acceptor of methanol dehydrogenase. Both methanol dehydrogenase and methylamine dehydrogenase contain PQQ as a cofactor. These so-called quinoproteins are able to catalyze redox reactions by one-electron steps. The reaction mechanism of this oxidation will be described. Electrons from the oxidation reaction are donated to the electron transport chain at the level of cytochrome
c. P. denitrificans is able to synthesize at least 10 different
c-type cytochromes. Five could be detected in the periplasm and five have been found in the cytoplasmic membrane. The membrane-bound cytochrome
c
1 and cytochrome
c
552 and the periplasmic-located cytochrome
c
550 are present under all tested growth conditions. The cytochromes
c
551i
and
c
553i
, present in the periplasm, are only induced in cells grown on methanol, methylamine, or choline. The other
c-type cytochromes are mainly detected either under oxygen limited conditions or under anaerobic conditions with nitrate as electron acceptor or under both conditions. An overview including the induction pattern of all
P. denitrificans c-type cytochromes will be given. The genes encoding cytochrome
c
1, cytochrome
c
550, cytochrome
c
551i
, and cytochrome
c
553i
have been isolated and sequenced. By using site-directed mutagenesis these genes were mutated in the genome. The mutants thus obtained were used to study electron transport during growth on C
1 compounds. This electron transport has also been studied by determining electron transfer rates in
in vitro experiments. The exact pathways, however, are not yet fully understood. Electrons from methanol dehydrogenase are donated to cytochrome
c
551i
. Further electron transport is either via cytochrome
c
550 or cytochrome
c
553i
to cytochrome
aa
3. However, direct electron transport from cytochrome
c
551i
to the terminal oxidase might be possible as well. Electrons from methylamine dehydrogenase are donated to amicyanin and then via cytochrome
c
550 to cytochrome
aa
3, but other routes are used also.
P. denitrificans is studied by several groups by using a genetic approach. Several genes have already been cloned and sequenced and a lot of mutants have been isolated. The development of a host/vector system and several techniques for mutation induction that are used in
P. denitrificans genetics will be described.
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