Award Date

12-1-2016

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Life Sciences

First Committee Member

Eduardo A. Robleto

Second Committee Member

Brian Hedlund

Third Committee Member

Jeffrey Shen

Fourth Committee Member

Ernesto Abel-Santos

Number of Pages

67

Abstract

Mutagenesis is central to the evolutionary process. We currently view evolution as a gradual process affecting all cells within a population. However, my project studied an underestimated part of the evolutionary process: mutations generated during stationary phase (caused by nutritional stress or growth arrest) within a subpopulation of cells. I used Bacillus subtilis, a bacterial model for cell growth and differentiation. Stationary-phase cultures of B. subtilis differentiate subpopulations that exhibit different survival strategies including competence, secondary metabolite production, biofilm formation, cannibalism, and endospore formation. The development of competence permits cells to uptake exogenous DNA and incorporate it into their genome. During competence, new alleles are acquired and recombined into the host’s genome which leads to genetic diversity. Hence, it has been proposed that the cells that develop competence are tasked with increasing genetic diversity, which increases the potential to restore growth and escape from stress. One key process in increasing genetic diversity in the competent subpopulation is the process of recombination. Through this process, cells acquire new gene alleles and genome rearrangements with the potential to increase fitness.

Interestingly, published results from my research group have indicated that increasing genetic diversity is not limited to the promotion of the recombination process and that a yet-to-be characterized process mediating the formation of point mutations is activated in conditions of stationary phase. This concept is supported by the following observations: i) defects in genetic factors that control competence (ComK and ComA) result in decreases in mutagenesis in non-growing cells; and ii) the observed decrease is independent of recombination. We speculate that a mechanism, activated during the competence (K-state) through the transcriptional activator ComK, is responsible for the formation of point mutations acquired in stationary phase. My project seeks to bring together these separate observations into a coherent understanding of how the K-state leads to increases in mutagenesis. We tested the hypothesis that the population of cells that enter the K-state (competence) experiences increased levels of mutagenesis during stationary phase. The increase in mutagenesis levels in K-cells was affected by reactive oxygen species. We concluded that K-cells activate error-prone repair of DNA lesions caused by reactive oxygen species.

Keywords

Bacillus subtilis; competence; mutagenesis; stationary phase

Disciplines

Microbiology

Language

English


Included in

Microbiology Commons

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