The human fungal pathogen Candida albicans switches from a budding yeast form to a polarized hyphal form in response to various external signals. This morphogenetic switching has been implicated in the development of pathogenicity. We have cloned the CaCDC35 gene encoding C. albicans adenylyl cyclase by functional complementation of the conditional growth defect of Saccharomyces cerevisiae cells with mutations in Ras1p and Ras2p. It has previously been shown that these Ras homologues regulate adenylyl cyclase in yeast. The C. albicans adenylyl cyclase is highly homologous to other fungal adenylyl cyclases but has less sequence similarity with the mammalian enzymes. C. albicans cells deleted for both alleles of CaCDC35 had no detectable cAMP levels, suggesting that this gene encodes the only adenylyl cyclase in C. albicans. The homozygous mutant cells were viable but grew more slowly than wild-type cells and were unable to switch from the yeast to the hyphal form under all environmental conditions that we analyzed in vitro. Moreover, this morphogenetic switch was completely blocked in mutant cells undergoing phagocytosis by macrophages. However, morphogenetic switching was restored by exogenous cAMP. On the basis of epistasis experiments, we propose that CaCdc35p acts downstream of the Ras homologue CaRas1p. These epistasis experiments also suggest that the putative transcription factor Efg1p and components of the hyphal-inducing MAP kinase pathway depend on the function of CaCdc35p in their ability to induce morphogenetic switching. Homozygous cacdc35 Delta cells were unable to establish vaginal infection in a mucosal membrane mouse model and were avirulent in a mouse model for systemic infections. These findings suggest that fungal adenylyl cyclases and other regulators of the cAMP signaling pathway may be useful targets for antifungal drugs.

In the yeast Saccharomyces cerevisiae the G‐protein beta gamma subunits have been shown to trigger downstream events of the pheromone response pathway. We have identified a new gene, designated STE20, which encodes a protein kinase homologue with sequence similarity to protein kinase C, which is required to transmit the pheromone signal from G beta gamma to downstream components of the signalling pathway. Overproduction of the kinase suppresses the mating defect of dominant‐negative G beta mutations providing genetic evidence for an interaction with G beta, and epistasis experiments show that this kinase functions after or at the same point as G beta gamma, but before any of the other currently identified components of the signalling pathway. This points to a potentially new mechanism of G‐protein mediated signal transduction, the activation of a protein kinase through G beta gamma.

The CST20 gene of Candida albicans was cloned by functional complementation of a deletion of the STE20 gene in Saccharomyces cerevisiae. CST20 encodes a homolog of the Ste20p͞p65 PAK family of protein kinases. Colonies of C. albicans cells deleted for CST20 revealed defects in the lateral formation of mycelia on synthetic solid ''Spider'' media. However, hyphal development was not impaired in some other media. A similar phenotype was caused by deletion of HST7, encoding a functional homolog of the S. cerevisiae Ste7p protein kinase. Overexpression of HST7 partially complemented the deletion of CST20. Cells deleted for CST20 were less virulent in a mouse model for systemic candidiasis. Our results suggest that more than one signaling pathway can trigger hyphal development in C. albicans, one of which has a protein kinase cascade that is analogous to the mating response pathway in S. cerevisiae and might have become adapted to the control of mycelial formation in asexual C. albicans.Candida albicans is the major fungal pathogen in humans, causing various forms of candidiasis. This fungus is diploid with no sexual cycle and is capable of a morphological transition from a unicellular budding yeast to a filamentous form. Extensive filamentous growth leads to the formation of a mycelium displaying hyphae with branches and lateral buds. In view of the observation that hyphae seem to adhere to and invade host tissues more readily than does the yeast form, the switch from the yeast to the filamentous form probably contributes to the virulence of this organism (for a review, see ref. 1).Like C. albicans, bakers yeast Saccharomyces cerevisiae is also a dimorphic organism capable of switching under certain nutritional conditions from a budding yeast to a filamentous form. Under the control of nutritional signals, diploid cells switch to pseudohyphal growth (2), and haploid cells to invasive growth (3).The similarities between the dimorphic switching of S. cerevisiae and C. albicans suggest that these morphological pathways may be regulated by similar mechanisms in both organisms. In S. cerevisiae, morphological transitions are controlled by signaling components that are also involved in the mating response of haploid cells (3, 4). The switch to pseudohyphal growth requires a transcription factor encoded by the STE12 gene, and a mitogenactivated protein (MAP) kinase cascade including Ste7p (a homolog of MAP kinase kinase or MEK), Ste11p (a MEK kinase homolog), and Ste20p (a MEK kinase kinase) (3, 4). The MAP kinases involved in this response are as yet unknown (3, 4).We have recently shown that C. albicans contains a functional homolog of Ste7p (Hst7p; ref. 5). Cph1p, a homolog of the Ste12p transcription factor, is required for hyphal growth of C. albicans under certain in vitro conditions (6). Here we show a similar requirement for Hst7p and Cst20p, a C. albicans homolog of the Ste20p protein kinase. We also show in a mouse model for systemic candidiasis that Cst20p plays a role in virulence, as judged from signifi...