Biology Reference
In-Depth Information
5.
Aromatoleum
aromaticum
EbN1:
Anaerobic
Degradation
Specialist
The denitrifying aquatic bacterium
Aromatoleum aromaticum
EbN1 was the fi rst pure culture demonstrated to anaerobically
degrade ethylbenzene (
50
). Its versatility is refl ected by the wide
range of utilized aromatic growth substrates.
A
.
aromaticum
EbN1
belongs to a betaproteobacterial cluster of anaerobic degradation
specialists. This
Aromatoleum
-
Azoarcus
-
Thauera
cluster consists
of more than 20 different strains degrading aromatic compounds,
alkanes, and monoterpenes under nitrate-reducing conditions and
dominated enrichment cultures with crude oil (
51
). The genome
of
A
.
aromaticum
EbN1 was the fi rst to be determined for a mem-
ber of this phylogenetic cluster and of anaerobic hydrocarbon
degraders in general (
25
). The genome sequence together with the
below described differential proteomic studies allowed reconstruct-
ing the complex network of
A
.
aromaticum
EbN1 for anaerobic
degradation of aromatic compounds (Fig.
5
).
5.1. Background
A
.
aromaticum
EbN1 is unique in anaerobically degrading
ethylbenzene and toluene via two fundamentally different path-
ways, converging at the central intermediate benzoyl-CoA.
Ethylbenzene is initially hydroxylated at the methylene carbon,
then dehydrogenated, and carboxylated, until acetyl-CoA is
thiolytically removed. In contrast, the methyl group of toluene is
radically added to fumarate, followed by modifi ed
5.2. Anaerobic Toluene
and Ethylbenzene
Degradation
-oxidation and
thiolytic removal of succinyl-CoA. Directed by Edman sequences
of 2DE-resolved substrate-specifi c proteins (
52
), the complete
gene clusters for both pathways were assembled from whole-genome
shotgun sequencing data (
53, 54
).
The substrate-dependent regulation of both pathways was
studied at the RNA (real-time RT-PCR and DNA microarray) and
protein (2D DIGE) level. The toluene-related
bss
and
bbs
operons
were specifi cally expressed in toluene-adapted cells and suggested
to be coordinately regulated by a single two-component system.
In contrast, ethylbenzene-related genes (
ebd
/
ped
and
apc
operons)
were expressed in ethylbenzene- and toluene-adapted cells, sug-
gesting gratuitous induction by toluene or cross talk between
the sensor/regulator systems of both pathways. Differential gene
expression and protein profi les of ethylbenzene- and acetophe-
none-adapted cells suggested a sequential regulation by two dis-
tinct two-component systems responsive to ethylbenzene and
acetophenone, respectively. Moreover, the profi les indicated
involvement of several thus far unknown proteins in the degrada-
tion of both alkylbenzenes (
55
).
β
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