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Elucidation of the metabolic pathway for dibenzothiophene desulphurization by Rhodococcus sp. strain IGTS8 (ATCC 53968)

Christopher Oldfield

Energy Biosystems Corporation, 4200 Research Forest Drive, The Woodlands, TX 77381, USA
Energy and Environmental Research Center, University of North Dakota, Grand Forks, ND 58202, USA
Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556-0309, USA

Author for correspondence: Christopher Oldfield. Tel: + 44 131 455 2217. Fax: +44 131 455 2291. e-mail: c.oldfield@napier.ac.uk

ABSTRACT

Summary: Rhodococcus sp. strain IGTS8 (ATCC 53968) is able to utilize dibenzothiophene (DBT) as a sole source of sulphur. The carbon skeleton of DBT is not metabolized and is conserved as 2-hydroxybiphenyl (HBP), which accumulates in the medium. This phenotype is due to the expression of the plasmid-encoded DBT-desulphurization (dsz) operon, which encodes three proteins, DszA, B and C. In this paper it is shown, using [³⁵S]DBT radiolabelling studies, that sulphur is released in the form of inorganic sulphite. The pathway of DBT desulphurization is described in detail. In summary, DszC catalyses the stepwise S-oxidation of DBT, first to dibenzothiophene 5-oxide (DBTO) and then to dibenzothiophene 5,5-dioxide (DBTO₂); DszA catalyses the conversion of DBTO₂ to 2-(2'-hydroxyphenyl)benzene sulphinate (HBPSi^-) and DszB catalyses the desulphination of HBPSi^- to give HBP and sulphite. Studies with cell-free extracts show that DszA and DszC, but not DszB, require NADH for activity. ¹⁸O₂-labelling studies show that each incorporated oxygen atom is derived directly from molecular oxygen. These results are consistent with the role of DszC as a mono-oxygenase, of DszA as an apparently unique enzyme which catalyses the reductive hydroxylation of DBTO₂ leading to cleavage of the thiophene ring, and of DszB as an aromatic sulphinic acid hydrolase.

Present address: Department of Biological Sciences, Napier University, Edinburgh EH 10 5DT, UK.

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