We identified a gene of the fungal pathogen Candida albicans, designated EFG1, whose high‐level expression stimulates pseudohyphal morphogenesis in the yeast Saccharomyces cerevisiae. In a central region the deduced Efg1 protein is highly homologous to the StuA and Phd1/Sok2 proteins that regulate morphogenesis of Aspergillus nidulans and S.cerevisiae, respectively. The core of the conserved region is homologous to the basic helix–loop–helix (bHLH) motif of eukaryotic transcription factors, specifically to the human Myc and Max proteins. Fungal‐specific residues in the bHLH domain include the substitution of an invariant glutamate, responsible for target (E‐box) specificity, by a threonine residue. During hyphal induction EFG1 transcript levels decline to low levels; downregulation is effected at the level of transcriptional initiation as shown by a EFG1 promoter–LAC4 fusion. A strain carrying one disrupted EFG1 allele and one EFG1 allele under the control of the glucose‐repressible PCK1 promoter forms rod‐like, pseudohyphal cells, but is unable to form true hyphae on glucose‐containing media. Overexpression of EFG1 in C.albicans leads to enhanced filamentous growth in the form of extended pseudohyphae in liquid and on solid media. The results suggest that Efg1p has a dual role as a transcriptional activator and repressor, whose balanced activity is essential for yeast, pseudohyphal and hyphal morphogenesis of C.albicans. Functional analogies between Efg1p and Myc are discussed.

External signals induce the switch from a yeast to a hyphal growth form in the fungal pathogen Candida albicans. We demonstrate here that the catalytic subunit of a protein kinase A (PKA) isoform encoded by TPK2 is required for internal signalling leading to hyphal differentiation. TPK2 complements the growth defect of a Saccharomyces cerevisiae tpk1‐3 mutant and Tpk2p is able to phosphorylate an established PKA‐acceptor peptide (kemptide). Deletion of TPK2 blocks morphogenesis and partially reduces virulence, whereas TPK2 overexpression induces hyphal formation and stimulates agar invasion. The defective tpk2 phenotype is suppressed by overproduction of known signalling components, including Efg1p and Cek1p, whereas TPK2 overexpression reconstitutes the cek1 but not the efg1 phenotype. The results indicate that PKA activity of Tpk2p is an important contributing factor in regulating dimorphism of C. albicans.

TPK1 and TPK2 encode both isoforms of protein kinase A (PKA) catalytic subunits in Candida albicans. Mutants lacking both TPK1 alleles showed defective hyphal morphogenesis on solid inducing media, whereas in liquid hypha, formation was affected slightly. In contrast, tpk2 mutants were only partially morphogenesis defective on solid media, whereas a strong block was observed in liquid. In addition, the yeast forms of tpk2– but not tpk1– mutants were completely deficient in invading agar. Because Tpk1p and Tpk2p differ in their N‐terminal domains of approximately 80–90 amino acids, while the catalytic portions are highly homologous, the functions of hybrid Tpk proteins with exchanged N‐terminal domains were tested. The results demonstrate that the catalytic portions mediate Tpk protein specificities with regard to filamentation, whereas agar invasion is mediated by the N‐terminal domain of Tpk2p. Homozygous tpk1 and tpk2 mutants grew normally; however, a tpk2 mutant strain containing a single regulatable TPK1 allele (PCK1p‐TPK1) at low expression levels was severely growth defective. It was completely blocked in hyphal morphogenesis and was stress resistant to high osmolarities or temperatures. Thus, both Tpk isoforms in C. albicans share growth functions but, unlike Saccharomyces cerevisiae isoforms, they have positive, specific roles in filament formation in different environments.

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.

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