All ETDs from UAB

Advisory Committee Chair

Eric J Sorscher

Advisory Committee Members

Stephen A Watts

John L Hartman

Aubrey E Hill

Document Type

Thesis

Date of Award

2013

Degree Name by School

Master of Science (MS) College of Arts and Sciences

Abstract

The appearance of genomic mutation is generally considered a random process, during which single nucleotide polymorphisms (SNPs) and other DNA alterations are selected and fixed by virtue of beneficial effects on fitness. Recent studies have provided examples of induced mutations in response to stress, including the suggestion of biased SNP formation that favors certain adaptations. This project analyzed the appearance and accumulation of mutations in S. cerevisiae following exposure to two lethal forms of selective pressure. A mutation/reversion cycle was established in which 5-fluoroorotic acid (5-FoA) served as an environmental stress to identify the functional loss of URA3 (a gene required to decarboxylate 5-FoA and necessary for 5-FoA cellular toxicity), followed by -URA growth (i.e. absence of uracil, which is lethal if URA3 function has been disabled) to detect restoration of URA3. During selection cycling, we observed a tendency towards increased drug resistance and appearance of yeast strains that could survive under these two otherwise lethal selective pressures. Such adaptation might be genetic or epigenetic, and might involve specific gene defects or epistatic interactions (i.e. among many distinct genes). A full genomic analysis revealed a logical pattern of mutations that help explain the phenotypes observed. In addition, when complete DNA sequencing of yeast tested here was compared to the published reference genome, we noted a mutation bias in the founder sample, suggesting the action of a strong selective pressure and possibly including a preference in the way SNPs are originally generated in yeast. One mechanism that might contribute to strong synonymous SNP bias likely involves a preponderance of transition polymorphisms, which would affect SNP formation and favor synonymous nucleotide replacement. The information described here will contribute to understanding mutation accrual in eukaryotic cells, as well as possible ramifications of biased patterns of mutation in yeast.

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