MULTIDRUG-RESISTANCE IN ASPERGILLUS-NIDULANS INVOLVES NOVEL ATP-BINDING CASSETTE TRANSPORTERS

Two single-copy genes, designated atrA and atrB (ATP-binding cassette transporter A and B), were cloned from the filamentous fungus Aspergil lus nidulans and sequenced. Based on the presence of conserved motifs and on hydropathy analysis, the products encoded by atrA and atr B can be regarded as novel members of the ATP-binding cassette (ABC) superf amily of membrane transporters. Both products share the same topology as the ABC transporters PDR5 and SNQ2 from Saccharomyces cerevisiae an d CDR1 from Candida albicans, which are involved in multidrug resistan ce of these yeasts. Significant homology also occurs between the ATP-b inding cassettes of AtrA and AtrB, and those of mammalian ABC transpor ters (P-glycoproteins). The transcription of atrA and, in particular, atrB in mycelium of A. nidulans is strongly enhanced by treatment with several drugs, including antibiotics, azole fungicides and plant defe nse toxins. The enhanced transcription is detectable within a few minu tes after drug treatment and coincides with the beginning of energy-de pendent drug efflux activity, reported previously in the fungus for az ole fungicides. Transcription of the atr genes has been studied in a w ild-type and in a series of isogenic strains carrying the imaA and/or imaB genes, which confer multidrug resistance to various toxic compoun ds such as the azole fungicide imazalil. atrB is constitutively transc ribed at a low level in the wild-type and in strains carrying imaA or imaB. Imazalil treatment enhances transcription of atrB to a similar e xtent in all strains tested. atrA, unlike atrB, displays a relatively high level of constitutive expression in mutants carrying imaB. Imazal il enhances transcription of atrA more strongly in imaB mutants, sugge sting that the imaB locus regulates atrA. Functional analysis demonstr ated that cDNA of atrB can complement the drug hypersensitivity associ ated with PDR5 deficiency in S. cerevisiae.