Date of Award

Spring 2011

Document Type

Restricted Thesis

Terms of Use

© 2011 Caitlin E. Russell. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts

Department

Biology Department

First Advisor

Stephen T. Miller

Abstract

Through the process known as quorum sensing (QS), bacteria communicate with one another via small signaling molecules termed autoinducers. The production and release of autoinducers into the extracellular environment allow bacteria to gauge population density and modify gene expression accordingly. The autoinducer-2 (AI-2) class of signaling molecules, which consists of a set of interconverting derivatives of 4,5-dihydroxy-2,3-pentadione (DPD), has been shown to facilitate interspecies bacterial communication. In enteric bacteria Escherichia coli and Salmonella typhimurium as well as a diverse number of other bacteria species, AI-2 activates expression of the lsr operon. Gene products of the lsr operon function to internalize and process the signaling molecule, thereby interfering with the signaling attempts of other nearby species. One such gene product, the enzyme LsrG, has been implicated in degrading AI-2. Specifically, it has been shown to catalyze the isomerization of Phospho-DPD to 3,4,4-trihydroxy-2-pentanone-5-phosphate. In order to further characterize the reaction catalyzed by LsrG, the crystal structure of E. coli LsrG was determined to a resolution of 1.8 A. The enzyme crystallizes as an alpha+beta barrel and shares a protein fold with several enzymes belonging to the cofactor-independent monooxygenase family. Identification of the putative active site in addition to sequence alignment with LsrG orthologs allowed for the identification of potentially catalytic residues. Activity assays probed the catalytic activity of four of these residues (H65, E32, N25, and H70) via single point mutants. In order to visualize the enzyme-substrate binding interaction and thereby possibly elucidate a reaction mechanism, attempts were made at co-crystallizing the enzyme-ligand complex. As AI-2-mediated QS mediates the production of virulence factors and biofilm development, understanding the AI-2-degrading mechanism of the LsrG reaction has promising applications, particularly in the biomedical field.

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