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...

Aspergillus fumigatus is an important pathogen of the immunocompromised host causing pneumonia and invasive disseminated disease with high mortality. Previously, we identified a mutant strain (white, W) lacking conidial pigmentation and, in addition, the conidia showed a smooth surface morphology, whereas wild-type (WT) conidia are grey-green and have a typical ornamentation. W conidia appeared to be less protected against killing by the host defence, e.g., were more susceptible to oxidants in vitro and more efficiently damaged by human monocytes in vitro than WT conidia. When compared to the WT, the W mutant strain showed reduced virulence in a murine animal model. Genetic analysis suggested that the W mutant carried a single mutation which caused all of the observed phenotypes. Here. we report the construction of a genomic cosmid library of A. fumigatus and its use for complementation of the W mutant. Transformation of the W mutant was facilitated by co-transformation with plasmid pHELP1 carrying the autonomously replicating ama1 sequence of A. nidulans which also increased the transformation efficiency of A. fumigatus by a factor of 10. Using this cosmid library a putative polyketide synthase gene, designated pksP (polyketide synthase involved in pigment biosynthesis) was isolated. The pksP gene has a size of 6660 bp. pksP consists of five exons separated by short (47-73 bp) introns. Its deduced open reading frame is composed of 2146 amino acids. The pksP gene complemented both the white phenotype and the surface morphology of the W mutant conidia to wild type. Whereas W mutant conidia caused a strong reactive oxygen species (ROS) release by polymorphonuclear leukocytes, the ability of pksP-complemented W mutant conidia to stimulate ROS release was significantly reduced and comparable to that of WT conidia. In addition, the complemented strains showed restored virulence in a mouse model.

Aldoximes are known as floral and vegetative plant volatiles but also as biosynthetic intermediates for other plant defense compounds. While the cytochrome P450 monooxygenases (CYP) from the CYP79 family forming aldoximes as biosynthetic intermediates have been intensively studied, little is known about the enzymology of volatile aldoxime formation. We characterized two P450 enzymes, CYP79D6v3 and CYP79D7v2, which are involved in herbivore-induced aldoxime formation in western balsam poplar (Populus trichocarpa). Heterologous expression in Saccharomyces cerevisiae revealed that both enzymes produce a mixture of different aldoximes. Knockdown lines of CYP79D6/7 in gray poplar (Populus 3 canescens) exhibited a decreased emission of aldoximes, nitriles, and alcohols, emphasizing that the CYP79s catalyze the first step in the formation of a complex volatile blend. Aldoxime emission was found to be restricted to herbivore-damaged leaves and is closely correlated with CYP79D6 and CYP79D7 gene expression. The semi-volatile phenylacetaldoxime decreased survival and weight gain of gypsy moth (Lymantria dispar) caterpillars, suggesting that aldoximes may be involved in direct defense. The wide distribution of volatile aldoximes throughout the plant kingdom and the presence of CYP79 genes in all sequenced genomes of angiosperms suggest that volatile formation mediated by CYP79s is a general phenomenon in the plant kingdom.