Factors influencing the nitrogen-source dependent flucytosine resistance in Cryptococcus species

Flucytosine (5-FC) is an antifungal drug commonly used for treatment of cryptococcosis and several other systemic mycoses. In fungal species, cytosine permease and cytosine deaminase are known to play major roles in flucytosine resistance by regulating uptake and deamination of 5-FC, respectively. Cryptococcus species have three paralogs of cytosine permease (FCY2, FCY3, and FCY4) and three paralogs of cytosine deaminase (FCY1, FCY5, and FCY6). Like in other fungi, we found Fcy1 and Fcy2 to be the primary cytosine deaminase and permease respectively in C. neoformans H99 (VNI), C. gattii R265 (VGIIa), and WM276 (VGI). However, when different amino acids were used as the sole nitrogen source, Fcy3 and Fcy6 functioned as the secondary cytosine permease and deaminase respectively in C. gattii. Transcriptome analysis using RNA-seq under three different nitrogen sources (NH4, Asn, and Pro) revealed that these permease and deaminase genes as well as two GATA family transcription factor genes, GAT1 and CIR1, were differentially expressed in WM276. In addition, deletion studies of GAT1 and CIR1 demonstrated that the expression of each permease and deaminase genes was under the regulation of GAT1 and CIR1 individually. Furthermore, the expression levels of FCY3 and FCY6 regulated by GAT1 and CIR1 contributed to the 5-FC susceptibility in fcy2Δ or fcy1Δ background under different amino acid conditions. This study offers an insight into the mechanism of 5-FC resistance in C. gattii which is orchestrated by cytosine permease and deaminase genes, and two GATA family transcription factor genes, GAT1 and CIR1, under diverse nitrogen conditions. Total RNAs from Cryptococcus gattii WM276 strain grown on YNB media with 2% glucose containing NH4, Asn, or Pro as sole nitrogen source. Three biological replicates were prepared for the RNA-seq.