Date of Graduation

12-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Cell & Molecular Biology (PhD)

Degree Level

Graduate

Department

Biological Sciences

Advisor

Jeffrey A. Lewis

Committee Member

Andrew Alverson

Second Committee Member

Burt Bluhm

Third Committee Member

Erica Westerman

Fourth Committee Member

Jim Coleman

Keywords

Ethanol stress, Natural variation, Oxidative stress, Salt stress, Stress, Yeast

Abstract

Natural environments are dynamic, and organisms must sense and respond to changing conditions. One common way organisms deal with stressful environments is through gene expression changes, allowing for stress acclimation and resistance. Variation in stress sensing and signaling can potentially play a large role in how individuals with different genetic backgrounds are more or less resilient to stress. However, the mechanisms underlying how gene expression variation affects organismal fitness is often obscure.

To understand connections between gene expression variation and stress defense phenotypes, we have been exploiting natural variation in Saccharomyces cerevisiae stress responses using a unique phenotype called acquired stress resistance, where cells that are pretreated with a sub-lethal dose of stress survive lethal high doses of stress. This response is observed in organisms ranging from bacteria to humans, though the specific mechanisms governing acquisition of higher stress resistance are poorly understood.

This dissertation explores the mechanistic underpinnings of natural variation in yeast stress responses and resistance, thus identifying strategies that I argue are likely conserved across diverse organisms. We first show that a commonly-used lab strain fails to acquire oxidative stress resistance when pretreated with ethanol, while a wild oak strain can. Using genetic mapping, we provided new evidence that Hap1p, heme-dependent transcription factor, was responsible for variation in this trait through the regulation of CTT1-encoding cytosolic catalase T— hydrogen peroxide scavenging enzyme. Interestingly, the lab strain can still acquire higher hydrogen peroxide resistance when pretreated with salt, and this cross protection requires CTT1. To determine whether CTT1 was universally required for acquired hydrogen peroxide resistance, we tested over a dozen diverse yeast strains and found a wide range of catalase dependency suggesting that acquired hydrogen peroxide resistance arises through multiple anti-oxidant defense strategies. We used transcriptional profiling to identify potential signaling pathways and transcription factors that regulate differentially-expressed modules of genes during salt or ethanol stress and potential compensatory oxidative stress proteins.

These experiments highlight the power of using yeast natural variation to uncover novel aspects of conserved signaling networks and stress defenses, providing a framework for understanding the mechanistic underpinnings of natural variation in other organisms.

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