Indole-3-Acetic Acid as a Quorum-sensing Molecule in Saccharomyces cerevisiae

Hunter, Ally

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Indole-3-Acetic Acid as a Quorum-sensing Molecule in Saccharomyces cerevisiae

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Fungal infections have large implications in agriculture and medicine, and there are few interventions available in the form of antifungal agents due to their toxicity to the host. Saccharomyces cerevisiae is an excellent model for pathogenic fungi because it is a well-studied, tractable organism and shares some traits with pathogenic fungi. Like most pathogenic fungi, S. cerevisiae is dimorphic and transitions from the benign yeast form to a filamentous form in which it produces psuedohyphae. Finding novel routes of suppressing dimorphic transition in a model like S. cerevisiae could lead to the discovery of new antifungal agents. Recently, quorum-sensing mechanisms have been under investigation as new avenues for microbial control. Quorum sensing is a signaling phenomenon that is well described in bacteria. It is the regulation of gene expression in response to cell density via the accumulation of small signaling molecules in the immediate environment. Indole-3-acetic acid (IAA) demonstrates some of the criteria for being a quorum-sensing molecule in S. cerevisiae. The purpose of this thesis was to further explore IAA as a quorum-sensing molecule in S. cerevisiae by demonstrating IAA production by the organism in culture. Radio-labeled tryptophan incorporation experiments followed by Thin Layer Chromatograph (TLC) analysis demonstrated that IAA is produced in culture by S. cerevisiae. A screen of a commercially available gene deletion library using the radio-labeled trypophan incorporation assay identified genes implicated in the IAA biosynthetic pathway. Some of these genes are homologous to those in an IAA pathway in the fungus Ustilago maydis. Further investigation of deletion strains of these candidate genes shows that Ald2 and Ald3, two aldehyde dehydrogenases, are involved in IAA production. The double mutant, ald2∆ald3∆, makes less IAA than wild type and is unable to demonstrate haploid invasive growth. This supports the idea that IAA biosynthesis in S. cerevisiae is necessary for morphological transition and that IAA could serve as a quorum-sensing molecule in S. cerevisiae with dimorphic transition as the quorum-sensing phenotype.

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