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Journal of Bacteriology, October 2000, p. 5580-5585, Vol. 182, No. 19
Département de Biologie
Moléculaire et Structurale/BBSI and CNRS UMR
50921 and Département de Recherche
Fondamentale sur la Matière
Condensée/SCIB/SCPM,3 CEA-Grenoble,
F-38054 Grenoble Cedex 9, France, and Division of
Microbiology, National Research Centre for Biotechnology, Braunschweig,
Germany2
Received 20 April 2000/Accepted 5 July 2000
The xylT gene product, a component of the xylene
catabolic pathway of Pseudomonas putida mt2, has been
recently characterized as a novel [2Fe-2S] ferredoxin which
specifically reactivates oxygen-inactivated catechol 2,3-dioxygenase
(XylE). In this study, three XylT-like proteins potentially involved in
the catabolism of naphthalene (NahT) or cresols (PhhQ and DmpQ) have
been overexpressed in Escherichia coli, purified, and
compared with respect to their biochemical properties and interaction
with XylE. The three XylT analogues show general spectroscopic
characteristics common to plant-type [2Fe-2S] ferredoxins as well as
distinctive features that appear to be typical for the XylT subgroup of
these proteins. The midpoint redox potentials of the PhhQ and DmpQ
proteins were
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Characterization of Three XylT-Like [2Fe-2S]
Ferredoxins Associated with Catabolism of Cresols or Naphthalene:
Evidence for Their Involvement in Catechol Dioxygenase
Reactivation
286 mV and
323 mV, respectively. Interestingly, all
purified XylT-like proteins promoted in vitro reactivation of XylE
almost as efficiently as XylT. The interaction of XylE with XylT and its analogues was studied by cross-linking experiments using the 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. A polypeptide band with
an Mr of 46,000, which corresponded to the
cross-linked product between one XylE subunit and one molecule of
ferredoxin, was obtained in all cases. The formation of the complex was
affected by ionic strength, indicating that electrostatic forces are
involved in the dioxygenase-ferredoxin interaction. In complementation experiments, plasmids expressing xylT or its analogues were
introduced into an XylT-null mutant of P. putida which is
unable to grow on p-methylbenzoate. All transconjugants
regained the wild-type phenotype, indicating that all analogues can
substitute for XylT in the in vivo reactivation of XylE. Our results
provide evidence for a subgroup of [2Fe-2S] ferredoxins with distinct
biochemical properties whose specific function is to reactivate
intrinsically labile extradiol ring cleavage dioxygenases involved in
the catabolism of various aromatic hydrocarbons.
*
Corresponding author. Mailing address: CEA-Grenoble,
DBMS/BBSI, F-38054 Grenoble Cedex 9, France. Phone: 33 (0)4.76.88.43.10. Fax: 33 (0)4.76.88.51.85. E-mail:
yjouanneau{at}cea.fr.
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