Identification of Genes Upregulated by the Transcription Factor Bcr1 That Are Involved in Impermeability, Impenetrability, and Drug Resistance of Candida albicans a/α Biofilms

Candida albicans forms two types of biofilm, depending upon the configuration of the mating type locus. Although architecturally similar, a/alpha biofilms are impermeable, impenetrable, and drug resistant, whereas a/a and alpha/alpha biofilms lack these traits. The difference appears to be the result of an alternative matrix. Overexpression in a/a cells of BCR1, a master regulator of the a/alpha matrix, conferred impermeability, impenetrability, and drug resistance to a/a biofilms. Deletion of BCR1 in a/alpha cells resulted in the loss of these a/alpha-specific biofilm traits. Using BCR1 overexpression in a/a cells, we screened 107 genes of interest and identified 8 that were upregulated by Bcr1. When each was overexpressed in a/a biofilms, the three a/alpha traits were partially conferred, and when each was deleted in a/alpha cells, the traits were partially lost. Five of the eight genes have been implicated in iron homeostasis, and six encode proteins that are either in the wall or plasma membrane or secreted. All six possess sites for O-linked and N-linked glycosylation that, like glycosylphosphatidylinositol (GPI) anchors, can cross-link to the wall and matrix, suggesting that they may exert a structural role in conferring impermeability, impenetrability, and drug resistance, in addition to their physiological functions. The fact that in a screen of 107 genes, all 8 of the Bcr1-upregulated genes identified play a role in impermeability, impenetrability, and drug resistance suggests that the formation of the a/alpha matrix is highly complex and involves a larger number of genes than the initial ones identified here.