Academic journal article Genetics

Genetic Networks Inducing Invasive Growth in Saccharomyces Cerevisiae Identified through Systematic Genome-Wide Overexpression

Academic journal article Genetics

Genetic Networks Inducing Invasive Growth in Saccharomyces Cerevisiae Identified through Systematic Genome-Wide Overexpression

Article excerpt

ABSTRACT The budding yeast Saccharomyces cerevisiae can respond to nutritional and environmental stress by implementing a morphogenetic program wherein cells elongate and interconnect, forming pseudohyphal filaments. This growth transition has been studied extensively as a model signaling system with similarity to processes of hyphal development that are linked with virulence in related fungal pathogens. Classic studies have identified core pseudohyphal growth signaling modules in yeast; however, the scope of regulatory networks that control yeast filamentation is broad and incompletely defined. Here, we address the genetic basis of yeast pseudohyphal growth by implementing a systematic analysis of 4909 genes for overexpression phenotypes in a filamentous strain of S. cerevisiae. Our results identify 551 genes conferring exaggerated invasive growth upon overexpression under normal vegetative growth conditions. This cohort includes 79 genes lacking previous phenotypic characterization. Pathway enrichment analysis of the gene set identifies networks mediating mitogen-activated protein kinase (MAPK) signaling and cell cycle progression. In particular, overexpression screening suggests that nuclear export of the osmoresponsive MAPK Hog1p may enhance pseudohyphal growth. The function of nuclear Hog1p is unclear from previous studies, but our analysis using a nuclear-depleted form of Hog1p is consistent with a role for nuclear Hog1p in repressing pseudohyphal growth. Through epistasis and deletion studies, we also identified genetic relationships with the G2 cyclin Clb2p and phenotypes in filamentation induced by S-phase arrest. In sum, this work presents a unique and informative resource toward understanding the breadth of genes and pathways that collectively constitute the molecular basis of filamentation.

THE budding yeast Saccharomyces cerevisiae is dimorphic, exhibiting both a unicellular growth form and a multicellular filamentous state generated presumably as a foraging mechanism under conditions of nutritional stress (Gimeno et al. 1992; Liu et al. 1993; Roberts and Fink 1994; Cook et al. 1996). In S. cerevisiae, nitrogen stress (Gimeno et al. 1992), growth in the presence of short-chain alcohols (Dickinson 1996; Lorenz et al. 2000a), and glucose stress (Cullen and Sprague 2000) can induce the transition to a filamentous form characterized morphologically as follows. Yeast cells undergoing filamentous growth are elongated in shape, due to delayed G2/M progression and prolonged apical growth (Gimeno et al. 1992; Kron et al. 1994; Ahn et al. 1999; Miled et al. 2001). Some reports indicate that these cells bud in a preferentially unipolar fashion (Gimeno et al. 1992; Kron et al. 1994), and, most distinctively during filamentous growth, daughter cells bud from mother cells but remain physically connected after septum formation (Gimeno et al. 1992). As a result, the interconnected cells form filaments that are termed pseudohyphae since they superficially resemble hyphae but lack the structure of a true hyphal tube with parallel-sided walls (Berman and Sudbery 2002). Depending on the induction condition and strain ploidy, pseudohyphal filaments can spread outward from a yeast colony over an agar surface and can also invade the agar (Gancedo 2001). This pseudohyphal growth response is not unique to S. cerevisiae; the related pathogenic fungus Candida albicans also exhibits pseudohyphal and hyphal morphologies, and the ability to switch between yeast, pseudohyphal, and hyphal growth forms is generally considered to be necessary for virulence in C. albicans (Braun and Johnson 1997; Lo et al. 1997; Jayatilake et al. 2006).

Pseudohyphal growth in S. cerevisiae is mediated by at least three well-studied signaling pathways encompassing the mitogen-activated protein kinase (MAPK) Kss1p, the AMP-activated kinase family member Snf1p, and cyclic AMP-dependent protein kinase A (PKA). The filamentous growth MAPK cascade consists of Ste11p, Ste7p, and Kss1p (Liu et al. …

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