Diazaborine resistance in yeast involves the efflux pumps Ycf1p and Flr1p and is enhanced by a gain-of-function allele of gene YAP1

We have found that YAP1-mediated diazaborine resistance in the yeast Saccha romyces cerevisiae requires two efflux pumps, i.e. the major-facilitator-su perfamily transporter Flr1p, which is located in the cytoplasmic membrane a nd the ATP-binding-cassette transporter Ycf1p which is present in the vacuo lar 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. Overexpressio n of YAP1 in a Delta flr1 Delta ycf1 double disruptant strain does not medi ate any diazaborine resistance, showing that these pumps are the only ones involved in detoxification of this drug. We also found a new mechanism of d iazaborine resistance which is caused by an allelic form of YAP1, designate d YAP1-11. This allele of YAP1 carries a mutation that leads to a C620F exc hange 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 resist ance. The protein encoded by the gain-of-function allele may transactivate by a different mechanism from the wild-type protein when overexpressed beca use it does not enhance YCF1 mRNA and still mediates diazaborine resistance in a Delta flr1 Delta ycf1 background.