No abstract

Phenotypic switching in Candida albicans spontaneously generates different cellular morphologies and is manifested in strain WO-1 by the reversible switching between the white and opaque phenotypes. We present evidence that phenotypic switching is regulated by the Efg1 protein, which is known as an essential element of hyphal development (dimorphism). Firstly, EFG1 is expressed specifically in cells of the white but not the opaque phenotype. During mass conversion from the opaque to the white phenotype, theEFG1 transcript level correlates with competence of switching of opaque cells to the white form. Secondly, overexpression of EFG1 by a PCK1p-EFG1 fusion forces opaque-phase cells to switch to the white form with a high level of efficiency. Thirdly, low-level expression of EFG1 in strain CAI-8 generates a cellular phenotype similar to that of opaque cells in that cells bud as short rods, which cannot be induced to form hyphae in standard conditions; such cells (unlike authentic opaque cells) lack typical surface “pimples.” Importantly, the opaque-specificOP4 transcript is induced in the opaque-like cells generated by strain CAI8 as a response to low-level expression ofEFG1. The results suggest that high EFG1expression levels induce and maintain the white cell form while lowEFG1 expression levels induce and maintain the opaque cell form. It is proposed that changes in EFG1 expression determine or contribute to phenotypic switching events in C. albicans.

Fungal APSES proteins regulate morphogenetic processes, including filamentation and differentiation. The human fungal pathogen Candida albicans contains two APSES proteins: the regulator Efg1p and its homologue Efh1p, described here. Overexpression of EFG1 or EFH1 led to similar phenotypes, including pseudohypha formation and opaque-white switching. An efh1 deletion generated no phenotype under most conditions but caused hyperfilamentation in an efg1 background under embedded or hypoxic conditions. This suggests cooperation of these APSES proteins in the suppression of an alternative morphogenetic signaling pathway. Genome-wide transcriptional profiling revealed that EFG1 and EFH1 regulate partially overlapping sets of genes associated with filament formation. Unexpectedly, Efg1p not only regulates genes involved in morphogenesis but also strongly influences the expression of metabolic genes, inducing glycolytic genes and repressing genes essential for oxidative metabolism. Using one-and two-hybrid assays, we further demonstrate that Efg1p is a repressor, whereas Efh1p is an activator of gene expression. Overall, the results suggest that Efh1p supports the regulatory functions of the primary regulator, Efg1p, and indicate a dual role for these APSES proteins in the regulation of fungal morphogenesis and metabolism.