Hyphal growth in the opportunistic fungal pathogen Candida albicans is believed to contribute to the virulence of the organism by promoting penetration of fungal cells into host tissue. In this study, stimulation of hyphal growth by a feature of the physical environment was demonstrated. Specifically, growth of cells embedded within a matrix promoted the formation of hyphae. The CZF1 gene, encoding a putative transcription factor, was shown to be involved in the regulation of hyphal growth under certain conditions, including embedded conditions. Ectopic expression of CZF1 in embedded cells promoted the rapid formation of hyphae. Elimination of CZF1 and CPH1, encoding a homologue of the Saccharomyces cerevisiae Ste12p transcription factor, led to a pronounced defect in filamentous growth of embedded cells. Elimination of CZF1 alone led to a moderate defect in hyphal growth under some conditions, including embedded conditions. Hyphal morphogenesis in response to matrix embedding may occur in the opportunistic pathogen, C. albicans, to promote invasion of fungal cells into host tissue.

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To allow the regulated expression of cloned genes in Candida albicans, a plasmid was constructed using the inducible promoter of the C. Albicans MAL2 gene. To demonstrate that the MAL2 promoter could regulate cloned genes placed under its control, a fusion construct was made with the coding sequence of the C. albicans URA3 gene. This plasmid was introduced into a Ura- strain of C. albicans using the process of restriction enzyme-mediated integration (REMI). This procedure involves the transformation of the BamHI-linearized plasmid in the presence of BamHI enzyme. The majority of transformants generated contained insertions of the plasmid at chromosomal BamHI sites. All transformants examined were inducible for URA3 expression, which was determined by growth analysis and by measuring the level of URA3 gene product activity. The URA+ phenotype of the transformants was stable during growth under nonselective conditions. This system offers the advantages of stable transformation, easy recovery of integrated DNA, and inducible expression of genes in C. albicans.

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