| Most fungal species have the ability to alternate between a unicellular yeast form and a filamentous form (dimorphism). Fungal dimorphism has received increasing attention because of its implication in pathogenesis and its potential as a simple experimental model of eukaryotic cell differentiation. To date, only a few species have been systematically investigated with an aim to understanding the molecular aspects of dimorphism. Studies have concentrated mainly on Candida albicans, the most common human fungal pathogen, and Saccharomyces cerevisiae, the most extensively studied fungus at the genetic, biochemical and physiological levels. In order to overcome some of the difficulties and shortcomings presented by these organisms, we have chosen to study dimorphic transition in Yarrowia lipolytica, because it can reproduce sexually, is amenable to genetic and molecular biological analysis, and its response to the induction of mycelial growth is highly reproducible.;This thesis describes the isolation and initial characterization of MHY1, YlRAC1, YlBEM1 and YlBMH1, four genes involved in the induction of hyphal growth in Y. lipolytica. These genes were isolated by their ability to restore hyphal growth to mutant Y. lipolytica strains obtained by chemical mutagenesis. We found that the transcript levels of these genes are increased during the yeast-to-hypha transition. MHY1 encodes a C2H2-type zinc finger protein, Mhy1p, which can bind putative cis-acting DNA stress response elements and appears to be concentrated in the nuclei of actively growing cells found at the hyphal tip, suggesting that Mhy1p may act as a transcription factor. The protein products of YlRAC1 and YlBEM1, in turn, appear to be involved in some aspects of cell polarization. YlBMH1 encodes a member of the 14-3-3 family of proteins whose exact function in the induction of filamentous growth is still unknown.;The identification of the YlCDC42, YlSEC31 and YlBMH2 genes is also described in this work. YlSEC31 encodes a putative component of COPII secretory vesicles and was isolated by its ability to enhance hyphal growth when overexpressed in wild-type Y. lipolytica cells. The roles of YlCDC42 and YlBMH2 in the induction of filamentous growth in Y. lipolytica remain undetermined. |
