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Degradation of chlorophenoxyacetic acids by the lignin-degrading fungus Dichomitus squalens

Department of Biochemistry and Molecular Biology, Oregon Graduate Institute of Science and Technology, PO Box 91000, Portland, OR 97291-1000, USA

*Author for correspondence: Michael H. Gold. Tel: +1 503 690 1076. Fax: + 1 503 690 1464. e-mail: mgold@admin.ogi.edu

ABSTRACT

Summary: We have examined the degradation of ¹⁴C ring- and side-chain-labelled 2,4,5-trichlorophenoxyacetic acid by Dichomitus squalens and Phanerochaete chrysosporium. The effects of Mn²⁺ on the degradation of these radiolabeled substrates by D. squalens and the effect of nitrogen limitation on their degradation by D. chrysosporium suggested that in both fungi, side-chain cleavage was catalysed by a mechanism independent of the lignin degradation system, whereas the degradation of the aromatic ring was dependent on the lignin degradative system. Using unlabelled substrates, a pathway for the degradation of chlorophenoxyacetic acids was elucidated in D. squalens. Time courses for the degradation of unlabelled chlorophenoxyacetic acids by D. squalens demonstrated that the corresponding chlorophenol was the initial product formed. The chlorophenol intermediate was xylosylated to form the chlorophenolxyloside. In turn, the chlorophenolxyloside could be hydrolysed by an intracellular -xylosidase to regenerate the chlorophenol. The chlorophenol product of the xylosidase reaction was oxidatively dechlorinated to form 2-chloro-p-benzoquinone which could undergo subsequent further dechlorination and ring-opening reactions, as has been reported previously for P. chrysosporium.

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