We have found that YAP1-mediated diazaborine resistance in the yeast Saccharomyces cerevisiae requires two efflux pumps, i.e. the major-facilitator-superfamily transporter Flr1p, which is located in the cytoplasmic membrane and the ATP-binding-cassette transporter Ycf1p which is present in the vacuolar membrane. Both these transporters are known to be under the control of the transcriptional transactivator Yap1p which explains our earlier finding that overexpression of YAP1 mediates diazaborine resistance. Overexpression of YAP1 in a Deltaflr1Deltaycf1 double disruptant strain does not mediate any diazaborine resistance, showing that these pumps are the only ones involved in detoxification of this drug. We also found a new mechanism of diazaborine resistance which is caused by an allelic form of YAP1, designated YAP1-11. This allele of YAP1 carries a mutation that leads to a C620F exchange in the C-terminal cysteine-rich-domain region and is the first mutant of YAP1 that was isolated by a conventional genetic screen for drug resistance. The protein encoded by the gain-of-function allele may transactivate by a different mechanism from the wild-type protein when overexpressed because it does not enhance YCF1 mRNA and still mediates diazaborine resistance in a Deltaflr1Deltaycf1 background.
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