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Journal of Bacteriology, December 2001, p. 6763-6770, Vol. 183, No. 23
Environmental Engineering and Science,
Department of Civil and Environmental
Engineering,1 and Department of
Biological Sciences,2 Stanford University,
Stanford, California 94305-4020
Received 18 May 2001/Accepted 23 August 2001
Biochemical studies in Azoarcus sp. strain T have
demonstrated that anaerobic oxidation of both toluene and
m-xylene is initiated by addition of the aromatic
hydrocarbon to fumarate, forming benzylsuccinate and 3-methyl
benzylsuccinate, respectively. Partially purified benzylsuccinate
synthase was previously shown to catalyze both of these addition
reactions. In this study, we identified and sequenced the genes
encoding benzylsuccinate synthase from Azoarcus sp.
strain T and examined the role of this enzyme in both anaerobic toluene
and m-xylene mineralization. Based on reverse
transcription-PCR experiments and transcriptional start site mapping,
we found that the structural genes encoding benzylsuccinate synthase,
bssCAB, together with two additional genes,
bssD and bssE, were organized in an
operon in the order bssDCABE. bssD is believed to encode an activating enzyme, similar in function to pyruvate formate-lyase activase. bssE shows homology to tutH
from Thauera aromatica strain T1, whose function is
currently unknown. A second operon that is upstream of
bssDCABE and divergently transcribed contains two genes,
tdiS and tdiR. The predicted amino acid
sequences show similarity to sensor kinase and response regulator
proteins of prokaryotic two-component regulatory systems. A chromosomal
null bssA mutant was constructed (the bssA
gene encodes the
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.23.6763-6770.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Benzylsuccinate Synthase of Azoarcus
sp. Strain T: Cloning, Sequencing, Transcriptional Organization, and
Its Role in Anaerobic Toluene and m-Xylene
Mineralization
-subunit of benzylsuccinate synthase). This
bssA null mutant strain was unable to grow under
denitrifying conditions on either toluene or m-xylene,
while growth on benzoate was unaffected. The growth phenotype of the
bssA mutant could be rescued by reintroducing bssA in trans. These results demonstrate
that benzylsuccinate synthase catalyzes the first step in anaerobic
mineralization of both toluene and m-xylene.
*
Corresponding author. Mailing address: Environmental
Engineering and Science, Department of Civil and Environmental
Engineering, Stanford University, Stanford, CA 94305-4020. Phone: (650)
723-3668. Fax: (650) 725-3164. E-mail:
spormann{at}stanford.edu.
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