Phosphorylation/dephosphorylation is likely the most crucial chemical reaction taking place in all living organisms. It is the basis for the regulatory control of many diverse biological events triggered by extracellular effectors. Moreover, it is a ubiquitous element of intracellular signal transduction pathways that regulates a wide range of processes. While protein phosphorylation has been extensively characterized in eukaryotes, far less is known about its emerging counterparts in prokaryotes. This study involved determination of the crystal structures and functional characterization of two protein kinases, YihE and AceK (also a protein phosphatase), and two nucleotide pyrophosphatases, YjjX and YhdE. X-ray crystallographic structure determination combined with bioinformatics analyses, mutageneses and biochemical experiments, both in vitro and in vivo, were utilized for the functional characterization of each protein. YihE was found to be a previously unknown kinase component of a new type of bacterial phospho-relay mechanism, thus adding kinase activity as another response to the Cpx sensing system that functions to maintain cellular homeostasis. AceK, which possesses both kinase and phosphatase activities, modifies isocitrate dehydrogenase (ICDH) to regulate the flux of isocitrate into the glyoxylate cycle. The structures of Acek alone and in complex with its substrate, ICDH, provided us with information to explain the mechanisms underlying its bifunctionality and its molecular switch. Through structural comparison and, particularly, functional characterization, we revealed that YjjX is a novel ITPase/XTPase responsible for the removal of non-canonical nucleotides from the cell during oxidative stress in Escherichia coli. YhdE, identified as a novel dTTPase, was observed to retard cell growth and form a filamentous phenotype when overexpressed in the cell, suggesting that YhdE is involved in the control of cell growth and division by regulating the cell nucleotide pool for DNA synthesis. In summary, this research has made a substantial ii contribution to the investigation of bacterial phosphorylation and dephophorylation systems that respond to various environmental conditions. / Thesis (Ph.D, Biochemistry) -- Queen's University, 2009-05-29 11:41:41.832