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J. Biol. Chem., Vol. 279, Issue 49, 51234-51240, December 3, 2004

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The Role of Thiol Species in the Hypertolerance of Aspergillus sp. P37 to Arsenic^*

David Cánovas¶, Riet Vooijs||, Henk Schat||, and Víctor de Lorenzo**

From the Centro Nacional de Biotecnología-Consejo Superior de Investigaciones Cientiíficas (CSIC), Universidad Autónoma de Madrid Campus de Cantoblanco, Madrid 28049, Spain, the ^||Department of Ecology and Physiology of Plants, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands, and the ^**Centro de Astrobiología (Instituto Nacional de Técnica Aeroespacial-CSIC), Torrejón de Ardoz, Madrid 28850, Spain

Aspergillus sp. P37 is an arsenate-hypertolerant fungus isolated from a river in Spain with a long history of contamination with metals. This strain is able to grow in the presence of 0.2 M arsenate, i.e. 20-fold higher than the reference strain, Aspergillus nidulans TS1. Although Aspergillus sp. P37 reduces As(V) to As(III), which is slowly pumped out of the cell, the measured efflux of oxyanions is insufficient to explain the high tolerance levels of this strain. To gain an insight into this paradox, the accumulation of acid-soluble thiol species in Aspergillus sp. P37 when exposed to arsenic was compared with that of the arsenic-sensitive A. nidulans TS1 strain. Increasing levels of arsenic in the medium did not diminish the intracellular pool of reduced glutathione in Aspergillus sp. P37, in sharp contrast with the decline of glutathione in A. nidulans under the same conditions. Furthermore, concentrations of arsenic that were inhibitory for the sensitive A. nidulans strain (e.g. 50 mM and above) provoked a massive formation of vacuoles filled with thiol species. Because the major fraction of the cellular arsenic was present as the glutathione conjugate As(GS)₃, it is plausible that the arsenic-hypertolerant phenotype of Aspergillus sp. P37 is in part due to an enhanced capacity to maintain a large intracellular glutathione pool under conditions of arsenic exposure and to sequester As(GS)₃ in vacuoles. High pressure liquid chromatography analysis of cell extracts revealed that the contact of Aspergillus sp. P37 (but not A. nidulans) with high arsenic concentrations (150 mM) induced the production of small quantities of a distinct thiol species indistinguishable from plant phytochelatin-2. Yet, we argue that phytochelatins do not explain arsenic resistance in Aspergillus, and we advocate the role of As(GS)3 complexes in arsenic detoxification.

Received for publication, July 29, 2004 , and in revised form, September 3, 2004.

^* This work was supported by a European Union BIOTOOL Contract (to V. d. L.) and by Comunidad Autónoma de Madrid Grant 07M/0075/2002 (to D. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

These two authors contributed equally to this paper.

^¶ Present address: Dept. of Genetics, University of Melbourne, Victoria 3010, Australia.

To whom correspondence should be addressed. Tel.: 34-91-585-4536; Fax: 34-91-585-4506; E-mail: vdlorenzo{at}cnb.uam.es.

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