Functional Portrait of Irf1 (Orf19.217), a Regulator of Morphogenesis and Iron Homeostasis in Candida albicans

Genetic Analysis of Candida albicans Filamentation by the Iron Chelator BPS Reveals a Role for a Conserved Kinase-WD40 Protein Pair

Candida albicans is a major human pathogenic fungus that is distinguished by its capability to switch from a yeast to a hyphal morphology under different conditions. Here, we analyze the cellular effects of high concentrations of the iron chelator bathophenanthroline disulfonate (BPS). BPS inhibits cellular growth by withholding iron, but when iron chelation is overcome by the addition of hemoglobin as an iron source, the cells resume growth as hyphae. The BPS hyphal induction pathway was characterized by identifying the hyphal-specific transcription factors that it requires and by a forward genetic screen for mutants that fail to form hyphae in BPS using a transposon library generated in a haploid strain. Among the mutants identified are the DYRK1-like kinase Yak1 and Orf19.384, a homolog of the DYRK1-associated protein WDR68/DCAF7. Orf19.384 nuclear localization depends on Yak1, similar to their mammalian counterparts. We identified the hyphal suppressor transcription factor Sfl1 as a candidate target of Yak1-Orf19.384 and show that Sfl1 modification is similarly affected in the yak1 and orf19.384 mutant strains. These results suggest that DYRK1/Yak1 and WDR68/Orf19.384 represent a conserved protein pair that regulates cell differentiation from fungi to animals.

Functional Dichotomy for a Hyphal Repressor in Candida albicans

Nrg1 is a repressor of hypha formation and hypha-associated gene expression in the fungal pathogen Candida albicans. It has been well studied in the genetic background of the type strain SC5314. Here, we tested Nrg1 function in four other diverse clinical isolates through an analysis of nrg1Δ/Δ mutants, with SC5314 included as a control. In three strains, nrg1Δ/Δ mutants unexpectedly produced aberrant hyphae under inducing conditions, as assayed by microscopic observation and endothelial cell damage. The nrg1Δ/Δ mutant of strain P57055 had the most severe defect. We examined gene expression features under hypha-inducing conditions by RNA-sequencing (RNA-Seq) for the SC5314 and P57055 backgrounds. The SC5314 nrg1Δ/Δ mutant expressed six hypha-associated genes at reduced levels compared with wild-type SC5314. The P57055 nrg1Δ/Δ mutant expressed 17 hypha-associated genes at reduced levels compared with wild-type P57055, including IRF1, RAS2, and ECE1. These findings indicate that Nrg1 has a positive role in hypha-associated gene expression and that this role is magnified in strain P57055. Remarkably, the same hypha-associated genes affected by the nrg1Δ/Δ mutation in strain P57055 were also naturally expressed at lower levels in wild-type P57055 than those in wild-type SC5314. Our results suggest that strain P57055 is defective in a pathway that acts in parallel with Nrg1 to upregulate the expression of several hypha-associated genes. IMPORTANCE Hypha formation is a central virulence trait of the fungal pathogen Candida albicans. Control of hypha formation has been studied in detail in the type strain but not in other diverse C. albicans clinical isolates. Here, we show that the hyphal repressor Nrg1 has an unexpected positive role in hypha formation and hypha-associated gene expression, as revealed by the sensitized P57055 strain background. Our findings indicate that reliance on a single type strain limits understanding of gene function and illustrate that strain diversity is a valuable resource for C. albicans molecular genetic analysis.