Negative regulation of Candida glabrata Pdr1 by the deubiquitinase subunit Bre5 occurs in a ubiquitin independent manner

Summary The primary route for development of azole resistance in the fungal pathogen Candida glabrata is acquisition of a point mutation in the PDR1 gene. This locus encodes a transcription factor that upon mutation drives high level expression of a range of genes including the ATP‐binding cassette transporter‐encoding gene CDR1 . Pdr1 activity is also elevated in cells that lack the mitochondrial genome (ρ ° cells), with associated high expression of CDR1 driving azole resistance. To gain insight into the mechanisms controlling activity of Pdr1, we expressed a tandem affinity purification (TAP)‐tagged form of Pdr1 in both wild‐type (ρ + ) and ρ ° cells. Purified proteins were analyzed by multidimensional protein identification technology mass spectrometry identifying a protein called Bre5 as a factor that co‐purified with TAP‐Pdr1. In Saccharomyces cerevisiae , Bre5 is part of a deubiquitinase complex formed by association with the ubiquitin‐specific protease Ubp3. Genetic analyses in C. glabrata revealed that loss of BRE5 , but not UBP3 , led to an increase in expression of PDR1 and CDR1 at the transcriptional level. These studies support the view that Bre5 acts as a negative regulator of Pdr1 transcriptional activity and behaves as a C. glabrata ‐specific modulator of azole resistance.