Collaboration between Antagonistic Cell Type Regulators Governs Natural Variation in the Candida albicans Biofilm and Hyphal Gene Expression Network

Candida albicans is among the most significant human fungal pathogens. However, the vast majority of C. albicans studies have focused on a single clinical isolate and its marked derivatives. We investigated natural variation among clinical C. albicans isolates in gene regulatory control of biofilm formation, a process crucial to virulence. The transcription factor Efg1 is required for biofilm-associated gene expression and biofilm formation. Previously, we found extensive variation in Efg1-responsive gene expression among 5 diverse clinical isolates. However, chromatin immunoprecipitation sequencing analysis showed that Efg1 binding to genomic loci was uniform among the isolates. Functional dissection of strain differences identified three transcription factors, Brg1, Tec1, and Wor1, for which small changes in expression levels reshaped the Efg1 regulatory network. Brg1 and Tec1 are known biofilm activators, and their role in Efg1 network variation may be expected. However, Wor1 is a known repressor of expression and an inhibitor of biofilm formation. In contrast, we found that a modest increase in RNA levels, reflecting the expression differences between C. albicans strains, could augment biofilm formation and expression of biofilm-related genes. The analysis of natural variation here reveals a novel function for a well-characterized gene and illustrates that strain diversity offers a unique resource for elucidation of network interactions. Clinical isolates of all pathogens vary in the strength of traits linked to disease. In this study, we focused on variation in a pathogenicity trait of the fungal pathogen Candida albicans, biofilm formation. This trait is under the control of the cell type regulator Efg1. Expression of Efg1 is known from previous studies to be repressed by a second cell type regulator, Wor1. However, we found that natural variation in biofilm formation and biofilm-related gene expression was driven by collaboration between Efg1 and Wor1. Our findings show that analysis of natural isolates can reveal unexpected features of gene function, even for well-studied genes.