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The ABC transporter AtrB from Aspergillus nidulans mediates resistance to all major classes of fungicides and some natural toxic compounds

Laboratory of Phytopathology, Wageningen University, PO Box 8025, 6700 EE Wageningen, The Netherlands¹
Institute of Plant Pathology, University of Naples ‘Federico II’, 80055 Portici, Naples, Italy²

Author for correspondence: Maarten A. De Waard. Tel: +31 317 483412. Fax: +31 317 483412. e-mail: maarten.dewaard{at}medew.fyto.wau.nl

This paper reports the functional characterization of AtrBp, an ABC transporter from Aspergillus nidulans. AtrBp is a multidrug transporter and has affinity to substrates belonging to all major classes of agricultural fungicides and some natural toxic compounds. The substrate profile of AtrBp was determined by assessing the sensitivity of deletion and overexpression mutants of atrB to several toxicants. All mutants showed normal growth as compared to control isolates. atrB mutants displayed increased sensitivity to anilinopyrimidine, benzimidazole, phenylpyrrole, phenylpyridylamine, strobirulin and some azole fungicides. Increased sensitivity to the natural toxic compounds camptothecin (alkaloid), the phytoalexin resveratrol (stilbene) and the mutagen 4-nitroquinoline oxide was also found. Overexpression mutants were less sensitive to a wide range of chemicals. In addition to the compounds mentioned above, decreased sensitivity to a broader range of azoles, dicarboximides, quintozene, acriflavine and rhodamine 6G was observed. Decreased sensitivity in overexpression mutants negatively correlated with levels of atrB expression. Interestingly, the overexpression mutants displayed increased sensitivity to dithiocarbamate fungicides, chlorothalonil and the iron-activated antibiotic phleomycin. Accumulation of the azole fungicide [¹⁴C]fenarimol by the overexpression mutants was lower as compared to the parental isolate, demonstrating that AtrBp acts by preventing intracellular accumulation of the toxicant. Various metabolic inhibitors increased accumulation levels of [¹⁴C]fenarimol in the overexpression mutants to wild-type levels, indicating that reduced accumulation of the fungicide in these mutants is due to increased energy-dependent efflux as a result of higher pump capacity of AtrBp.

Keywords: multidrug resistance, efflux pumps, ABC transporters, AtrBp, Aspergillus nidulans

Abbreviations: ABC, ATP-binding cassette; CCCP, carbonyl cyanide m-chlorophenylhydrazone; EC₅₀, effective concentration required for 50% growth inhibition; MDR, multidrug resistance; 4NQO, 4-nitroquinoline oxide; UTR, untranslated region

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