Impact of <i>Candida albicans NDT80</i> and <i>UME6</i> on biofilm formation and fluconazole susceptibility

The microbiome-associated fungus <i>Candida albicans</i> is an opportunistic pathogen. Virulence traits include its ability to produce biofilm, a surface-associated growth form that persists on mucosae and implanted medical devices. <i>C. albicans</i> clinical isolates vary in ability to produce biofilm and the constituent filamentous cell types. Here, we focus on two transcription factors that promote filamentation and biofilm formation, Ndt80 and Ume6. We address two questions. First, how variable is the impact of Ndt80 among <i>C. albicans</i> strains? Second, what is the genetic interaction between <i>NDT80</i> and <i>UME6</i>? We find that Ndt80 is required for filamentation and biofilm formation in five clinical isolates in addition to the reference strain SC5314, where Ndt80 function has been well established. RNA-sequencing (RNA-seq) data indicate that <i>UME6</i> RNA levels are reduced in an <i>ndt80</i>Δ/Δ mutant, possibly a result of altered <i>RME1</i> and <i>WOR1</i> expression, both of which control <i>UME6</i>. Increased expression of <i>UME6</i> in <i>ndt80</i>Δ/Δ mutants of three strain backgrounds restores filamentation and biofilm formation, though RNA-seq assays indicate that it does not suppress the overall <i>ndt80</i>Δ/Δ gene expression defect. Ndt80 has an additional role in promoting tolerance to the antifungal drug fluconazole, an inhibitor of ergosterol synthesis. This <i>ndt80</i>Δ/Δ phenotype varies considerably among clinical isolates. In three strains tested, increased expression of <i>UME6</i> in <i>ndt80</i>Δ/Δ mutants enhances their susceptibility to fluconazole. Therefore, our results show an unexpected relationship between Ume6 expression and azole drug sensitivity. To our knowledge, Ume6 has previously been understood to function only in filamentation, biofilm formation, and related processes.IMPORTANCEOur focus is the fungal pathogen <i>Candida albicans</i>. Two traits, biofilm/hypha formation and azole resistance, are major drivers of its infection ability. We examine the roles of two biofilm transcriptional regulators, Ndt80 and Ume6, in several <i>C. albicans</i> clinical isolates. Prior studies in one strain background (SC5314) indicated that Ndt80 controls both biofilm/hypha formation and azole drug susceptibility and that Ume6 controls biofilm/hypha formation. The four new findings here are that (i) Ndt80 effects on fluconazole sensitivity vary considerably with strain background; (ii) Ndt80 is required for filamentation and biofilm formation in multiple clinical isolates; (iii) the Ndt80 target Ume6 contributes to Ndt80 control of filamentation and biofilm formation in multiple clinical isolates; and (iv) Ume6 influences fluconazole vulnerability, the first Ume6 function to our knowledge that is unrelated to filamentation.