Diss. (sammanfattning) Umeå : Univ., 2006. / Härtill 3 uppsatser.

by Ng Wai-Leung. / Thesis submitted in: August 1997. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 135-144). / Title page --- p.i / Thesis Committee --- p.ii / Abstract --- p.iii / Acknowledgments --- p.v / Abbreviations --- p.vi / Table of contents --- p.vii / List of figures --- p.x / List of tables --- p.xiii / Chapter Chapter 1. --- General introduction --- p.1 / Chapter Chapter 2. --- Literature review / Chapter 2.1 --- Biosynthesis of deoxyribonucleotide triphosphates --- p.3 / Chapter 2.2 --- Ribonucleotide reductase --- p.6 / Chapter 2.2.1 --- Class I ribonucleotide reductase --- p.6 / Chapter 2.2.2 --- Class II ribonucleotide reductase --- p.13 / Chapter 2.2.3 --- Class III ribonucleotide reductase --- p.14 / Chapter 2.3 --- Proposed mechanism for ribonucleotide reduction --- p.17 / Chapter 2.4 --- Allosteric control of ribonucleotide reductase --- p.21 / Chapter 2.4.1 --- Allosteric control of class I ribonucleotide reductase --- p.21 / Chapter 2.4.2 --- Allosteric control of class II and class III ribonucleotide reductases --- p.23 / Chapter 2.5 --- Evolution of ribonucleotide reductase --- p.25 / Chapter 2.6 --- Central metabolism pathways of enteric bacteria --- p.28 / Chapter 2.7 --- Regulation of gene expression by oxygen in enteric bacteria --- p.33 / Chapter 2.7.1 --- Regulation of gene expression by Fnr --- p.33 / Chapter 2.7.2 --- Regulation of gene expression by AcrAB --- p.39 / Chapter 2.7.3 --- Regulation of gene expression by NarXL and NarQP --- p.42 / Chapter 2.7.4 --- Other aspects in anaerobic gene expression --- p.45 / Chapter 2.7.5 --- Relationship between NrdD and anaerobic metabolism --- p.45 / Chapter 2.8 --- Objectives --- p.46 / Chapter Chapter 3. --- Molecular cloning and sequencing of nrdDG operon from Salmonella typhimurium / Chapter 3.1 --- Introduction --- p.47 / Chapter 3.2 --- Material and methods --- p.48 / Chapter 3.2.1 --- Bacterial strains and bacteriophages --- p.48 / Chapter 3.2.2 --- Culture media --- p.48 / Chapter 3.2.3 --- Preparation of lambda lysate and phage DNA --- p.48 / Chapter 3.2.3.1 --- Plating out pf lambda phage and preparation of plate lysate --- p.48 / Chapter 3.2.3.2 --- Preparation of lambda lysate stock --- p.49 / Chapter 3.2.3.3 --- Preparation of lambda phage DNA --- p.50 / Chapter 3.2.4 --- Long distance polymerase chain reaction (LD-PCR) of nrdDG gene fragment --- p.51 / Chapter 3.2.5 --- Restriction enzyme digestion of LD-PCR products and subcloning of restriction fragments --- p.52 / Chapter 3.2.6 --- Confirmation of recombinants by colony-PCR --- p.53 / Chapter 3.2.7 --- Preparation of plasmid DNA by alkaline lysis using Wizard´ёØ Plus Miniprep DNA Purification System (Promega) --- p.54 / Chapter 3.2.8 --- DNA cycle sequencing by using dye-labeled dideoxy chain terminator and data collection --- p.55 / Chapter 3.2.9 --- Computer software for analyzing and manipulating DNA sequences --- p.57 / Chapter 3.3 --- Results --- p.59 / Chapter 3.3.1 --- Preparation of lambda DNA --- p.59 / Chapter 3.3.2 --- Long distance PCR amplification of nrdDG from lambda DNA --- p.59 / Chapter 3.3.3 --- Restriction digestion of LD-PCR products --- p.61 / Chapter 3.3.4 --- Subcloning of LD-PCR restriction fragments --- p.61 / Chapter 3.3.5 --- Miniprep of plasmid DNA from recombinants and verification of nrdDG identities --- p.64 / Chapter 3.3.6 --- Nucleotide sequence of nrdDG --- p.66 / Chapter 3.4 --- Discussions --- p.72 / Chapter 3.4.1 --- Sequence analysis of S. typhimurium nrdDG --- p.72 / Chapter 3.4.2 --- Experimental design --- p.79 / Chapter Chapter 4. --- Transcriptional regulation of anaerobic ribonucleotide reductase in Salmonella typhimurium in aerobic and anaerobic environments / Chapter 4.1 --- Introduction --- p.84 / Chapter 4.2 --- Materials and methods --- p.86 / Chapter 4.2.1 --- Bacteria and bacteriophages strains / Chapter 4.2.2 --- Culture media --- p.86 / Chapter 4.2.3 --- Construction and characterization of oxrA mutant --- p.87 / Chapter 4.2.3.1 --- Preparation of P22 lysate of TN2336 --- p.87 / Chapter 4.2.3.2 --- P22 transduction for construction of oxrA mutant --- p.87 / Chapter 4.2.3.3 --- Characterization of oxrA mutant --- p.87 / Chapter 4.2.4 --- Extraction of bacterial RNA by hot phenol method --- p.88 / Chapter 4.2.5 --- Formaldehyde gel electrophoresis of RNA --- p.88 / Chapter 4.2.6 --- Reverse transcriptase polymerase chain reaction (RT-PCR) of nrdD transcript --- p.89 / Chapter 4.2.7 --- Transfer of DNA/RNA to solid support --- p.90 / Chapter 4.2.7.1 --- Transfer of DNA to solid support by Southern blotting --- p.90 / Chapter 4.2.7.2 --- Transfer of RNA to solid support by Northern blotting --- p.91 / Chapter 4.2.7.3 --- RNA Dot blot --- p.91 / Chapter 4.2.8 --- Preparation of radioactive-labeled probes for hybridization --- p.92 / Chapter 4.2.8.1 --- Synthesis of radioactive-labeled probes by labeling --- p.92 / Chapter 4.2.8.2 --- Preparation of RNA probe by in vitro transcription --- p.93 / Chapter 4.2.9 --- Hybridization and membrane washing conditions --- p.95 / Chapter 4.2.10 --- Normalization of samples by 16S ribosomal RNA (rRNA) --- p.95 / Chapter 4.3 --- Results --- p.97 / Chapter 4.3.1 --- Preparation of RNA --- p.97 / Chapter 4.3.2 --- RT-PCR of nrdD transcript --- p.97 / Chapter 4.3.3 --- Northern blot analysis of nrdD transcript --- p.103 / Chapter 4.3.4 --- Dot blot hybridization analysis of nrdD expression in an oxrA mutant --- p.103 / Chapter 4.4 --- Discussions --- p.107 / Chapter 4.4.1 --- Expression of nrdD of S. typhimurium in aerobic and anaerobic environments --- p.107 / Chapter 4.4.2 --- Experimental design --- p.110 / Chapter Chapter 5. --- Characterization of nrdD::Tn10 mutant of S. typhimurium / Chapter 5.1 --- Introduction --- p.112 / Chapter 5.2 --- Materials and methods --- p.112 / Chapter 5.2.1 --- Bacteria and bacteriophages strains --- p.113 / Chapter 5.2.2 --- Transduction of zzz-3875::Tn10 to S. typhimurium --- p.113 / Chapter 5.2.3 --- Characterization of zzz-3875::Tn10 by Southern hybridization --- p.113 / Chapter 5.2.3.1 --- Preparation of genomic DNA from S. typhimurium --- p.113 / Chapter 5.2.3.2 --- Restriction enzyme digestion of genomic DNA and Southern hybridization --- p.114 / Chapter 5.2.4 --- Characterization of growth pattern of nrdD::Tn10 mutant --- p.115 / Chapter 5.3 --- Results --- p.116 / Chapter 5.3.1 --- Characterization of zzz-3 875: :Tn7 0 in S. typhimurium --- p.116 / Chapter 5.3.2 --- Characterization of growth pattern of nrdD mutant --- p.120 / Chapter 5.4 --- Discussions --- p.125 / Chapter Chapter 6. --- General Discussions / Chapter 6.1 --- General discussions --- p.131 / Chapter 6.2 --- Further studies --- p.134 / References --- p.135

食物中毒感染是常見但非常重要的全球性公共健康問題。沙門氏鼠傷寒桿菌乃常被分離出來的細菌性病原體之一。隨著實驗室參考菌株LT2的基因組序列於2001年被發表之後，另外9個沙門氏鼠傷寒菌菌株的基因序列均已陸續進行測序。最近，本實驗室亦對十個本地沙門氏鼠傷寒菌臨床分離菌株的基因序列進行了測序。為了為這些基因組序列提供高品質的註釋，我們把預測的基因組提交到質量控制工具GenePRIMP以識別有潛在錯誤或異常的預測基因。本研究針對血液分離菌株78896和糞便分離菌株1047518的GenePRIMP報告進行人工檢查，並對每個菌株超過270個的基因進行了修訂。此外，本研究亦對上述的10個本地菌株進行了功能註釋。註釋項目包括沙門氏菌致病島（SPIs）、致病因子、tRNA和非編碼小分子RNA、噬菌體和CRISPRs結構等基因組及致病元素。 KEGG通路則提供了進一步的功能註釋。 / 本研究同時對本地的血液和糞便分離菌株，連同國外的臨床分離菌株，進行了廣泛的比對，用以識別全身性沙門氏菌感染的潛在遺傳因素。 本研究進行了以下基因分析：（1）多位點序列分型（MLST）；（2）在小鼠全身性感染中涉及的主調控因子和關鍵元素; 及（3）人類腸胃道感染中涉及的基因。然而，這些分析產生只能對全身性沙門氏菌感染提供有限的見解。然而，透過使用RAST註釋系統，我們於其中三個血液分離菌株中發現了一個的額外的螯鐵蛋白aerobactin鐵採集系統。儘管在體外實驗中，這些血液分離菌株並沒有明顯的生長優勢，但實驗結果表明，在缺乏鐵的培養液中，aerobactin基因的表達水平是比較高的。此外，我們亦於其中四個血液分離菌株中，發現負責細胞色素c熟成(ccm)的基因座均被中斷。這可能改變了這些血液分離菌株中細胞色素c的生物合成途徑。這些鐵採集和同化機制的觀察均為未來全身性沙門氏菌感染的研究提供了可能的發展方向。 / 本研究同時識別了用以分別本地及海外的沙門氏鼠傷寒菌菌株的分子標記，並在鮭魚和生菜的接種實驗中，展現了它們分辨本地及海外菌株的能力。然而，在投入實際應用之前，這些標記尚需要進一步的驗證和測試，以便確定快速檢測方法的有效性。 / Foodborne infection is a common but important public health issue worldwide. Salmonella enterica serovar Typhimurium is frequently isolated from outbreaks as one of the common bacterial causative agents. Following the availability of the genome sequence of the reference lab strain LT2 in 2001, nine genomes of S. Typhimurium had been sequenced since then. Recently, genomes of ten local S. Typhimurium clinical isolates have been assembled in our laboratory. In order to provide high quality annotation of these genome sequences, the predicted gene sets were submitted to the quality control tool GenePRIMP (Gene PRediction IMprovement Pipeline) to identify potentially erroneous and abnormal gene calls. The GenePRIMP reports for the local blood isolate 78896 and stool isolate 1047518 were manually inspected and more than 270 genes were amended individually for each isolate. Functional annotation had also been performed for the 10 local isolates. Genomic and virulent elements including Salmonella Pathogenicity Islands (SPIs), virulence factors, tRNAs and small non-coding RNAs, prophage elements and CRISPRs structures had been annotated. The KEGG pathways provided a further means of functional annotation. / The local blood and stool isolates, together with the sequenced foreign clinical isolates, had also been extensively compared to identify potential genetic determinants of Salmonella systemic infection. (1) Multilocus sequence typing (MLST); (2) Alignment of master regulators and key players of systemic infection in mice; and (3) Analyses of the genes responsible for human gastrointestinal tract infection had been performed. However, these analyses yielded limited insights on systemic infection. Alternatively, using subsystems annotation by RAST, an additional aerobactin siderophore iron acquisition system was shown to be prevalent among three of the blood isolates. Despite no obvious growth advantage was offered to the blood isolates in an in vitro experiment, it was demonstrated that expression of the aerobactin genes was higher in iron-depleted culturing medium. In addition, a disrupted cytochrome c maturation (ccm) locus that may alter the cytochrome c biogenesis pathway was also identified in four of the blood isolates. These observations in iron acquisition and assimilation mechanisms suggest their potential in future direction of Salmonella systemic infection studies. / Molecular markers specific to local and foreign S. Typhimurium isolates were also identified and their utility in differentiating local and foreign isolates was demonstrated in a pilot spiking experiment using raw salmon and lettuce. These markers will require further verification and testing prior to actual application in real-world settings in order to examine the validity of the rapid detection method. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Cheng, Chi Keung. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 124-146). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / Abstract of thesis entitled --- p.iii / 摘要 --- p.v / Acknowledgements --- p.vii / Table of Contents --- p.viii / List of Tables --- p.xi / List of Figures --- p.xiii / Abbreviations --- p.xiv / Chapter Chapter 1 --- Literature Review --- p.1 / Chapter 1.1 --- Introduction and Taxonomy --- p.1 / Chapter 1.2 --- Epidemiology of Salmonella Typhimurium infections --- p.2 / Chapter 1.3 --- Pathogenesis of Salmonella Typhimurium infection --- p.4 / Chapter 1.3.1 --- Infection mechanisms --- p.4 / Chapter 1.3.2 --- Salmonella Pathogenicity Islands --- p.6 / Chapter 1.3.3 --- Regulation of virulence --- p.9 / Chapter 1.4 --- Non-typhoid Salmonella (NTS) systemic infection --- p.11 / Chapter 1.4.1 --- Epidemiology of NTS systemic infection --- p.11 / Chapter 1.4.2 --- Salmonella Typhimurium multidrug resistance --- p.12 / Chapter 1.5 --- Salmonella Typhimurium genomics --- p.15 / Chapter 1.5.1 --- Salmonella Typhimurium genome sequencing --- p.15 / Chapter 1.5.2 --- Comparative studies on Salmonella genomes --- p.17 / Chapter 1.6 --- Aims of project --- p.19 / Chapter Chapter 2 --- Curation and detailed annotation of genomes of local Salmonella Typhimurium clinical isolates --- p.22 / Chapter 2.1 --- Introduction --- p.22 / Chapter 2.2 --- Materials and Methods --- p.27 / Chapter 2.2.1 --- Manual curation of GenePRIMP results --- p.27 / Chapter 2.2.2 --- Salmonella Pathogenicity Islands (SPIs) and virulence factors annotation --- p.29 / Chapter 2.2.3 --- Small RNA and t-RNA annotation --- p.29 / Chapter 2.2.4 --- Phage elements annotation --- p.30 / Chapter 2.2.5 --- CRISPRs annotation --- p.30 / Chapter 2.2.6 --- KEGG annotation --- p.30 / Chapter 2.3 --- Results --- p.32 / Chapter 2.3.1 --- Manual curation of GenePRIMP results --- p.32 / Chapter 2.3.1.1 --- Short genes --- p.35 / Chapter 2.3.1.2 --- Long genes --- p.35 / Chapter 2.3.1.3 --- Unique genes --- p.36 / Chapter 2.3.1.4 --- Overlapped genes --- p.36 / Chapter 2.3.1.5 --- Broken genes --- p.37 / Chapter 2.3.2 --- Salmonella Pathogenicity Islands (SPIs) and virulence factors annotation --- p.37 / Chapter 2.3.2.1 --- Salmonella Pathogenicity Islands (SPIs) annotation --- p.37 / Chapter 2.3.2.2 --- Virulence factors annotation --- p.44 / Chapter 2.3.3 --- Small RNA and t-RNA annotation --- p.44 / Chapter 2.3.4 --- Phage elements annotation --- p.44 / Chapter 2.3.5 --- CRISPRs annotation --- p.50 / Chapter 2.3.6 --- KEGG annotation --- p.51 / Chapter 2.4 --- Discussion --- p.53 / Chapter 2.4.1 --- Manual curation of GenePRIMP results --- p.53 / Chapter 2.4.1.1 --- Gene amendment not required --- p.54 / Chapter 2.4.1.2 --- Genes with boundaries relocated --- p.54 / Chapter 2.4.1.3 --- Genes to be discarded --- p.55 / Chapter 2.4.1.4 --- Gene pairs to be fused --- p.55 / Chapter 2.4.1.5 --- Potential pseudogenes formation --- p.56 / Chapter 2.4.2 --- Salmonella Pathogenicity Islands (SPIs) annotation --- p.57 / Chapter 2.4.3 --- Virulence factors annotation --- p.57 / Chapter 2.4.4 --- Small RNA and t-RNA annotation --- p.58 / Chapter 2.4.5 --- Phage elements annotation --- p.59 / Chapter Chapter 3 --- Identification of genetic determinants of blood invasiveness in local S. Typhimurium clinical isolates --- p.61 / Chapter 3.1 --- Introduction --- p.61 / Chapter 3.2 --- Materials and Methods --- p.66 / Chapter 3.2.1 --- Multilocus Sequence Typing (MLST) --- p.66 / Chapter 3.2.2 --- Phage elements annotation for foreign isolates --- p.67 / Chapter 3.2.3 --- Alignment of genes inferred to play important roles in NTS systemic --- p.infection67 / Chapter 3.2.4 --- Alignment of genes inferred to involved during infection in the gastrointestinal (GI) tract --- p.68 / Chapter 3.2.5 --- Subsystems assignment using Rapid Annotation using Subsystem Technology (RAST) server --- p.68 / Chapter 3.2.6 --- Growth analysis of local S. Typhimurium clinical isolates in iron-limiting environment --- p.69 / Chapter 3.2.7 --- Reverse transcription and real-time PCR --- p.70 / Chapter 3.2.7.1 --- Primer design and verification --- p.70 / Chapter 3.2.7.2 --- cDNA synthesis and real-time PCR --- p.70 / Chapter 3.3 --- Results --- p.73 / Chapter 3.3.1 --- Multilocus Sequence Typing (MLST) --- p.73 / Chapter 3.3.2 --- Phage elements annotation for foreign isolates --- p.73 / Chapter 3.3.3 --- Alignment of genes inferred to play important roles in NTS systemic infection --- p.74 / Chapter 3.3.4 --- Alignment of genes inferred to involved during infection in the gastrointestinal (GI) tract --- p.79 / Chapter 3.3.4.1 --- Acid tolerance response --- p.79 / Chapter 3.3.4.2 --- Epithelial cells attachment --- p.80 / Chapter 3.3.4.3 --- Epithelial cells invasion --- p.83 / Chapter 3.3.4.4 --- Survival within macrophages --- p.83 / Chapter 3.3.5 --- RAST subsystem analysis --- p.86 / Chapter 3.3.6 --- Growth analysis and aerobactin genes expression --- p.87 / Chapter 3.4 --- Discussion --- p.93 / Chapter Chapter 4 --- Molecular markers identification and testing on selected foodstuff for local S. Typhimurium isolates --- p.97 / Chapter 4.1 --- Introduction --- p.97 / Chapter 4.2 --- Materials and Methods --- p.101 / Chapter 4.2.1 --- Molecular markers identification --- p.101 / Chapter 4.2.2 --- Primer design and verification --- p.101 / Chapter 4.2.3 --- Spiking experiments on selected food samples --- p.103 / Chapter 4.2.4 --- Quantitative TaqMan real-time PCR --- p.103 / Chapter 4.3 --- Results --- p.105 / Chapter 4.3.1 --- Molecular markers identification --- p.105 / Chapter 4.3.2 --- Spiking experiments and TaqMan real-time PCR --- p.109 / Chapter 4.4 --- Discussion --- p.113 / Chapter 4.4.1 --- Molecular markers identification --- p.113 / Chapter 4.4.2 --- Spiking experiments and TaqMan real-time PCR --- p.114 / Chapter Chapter 5 --- General discussion --- p.116 / Chapter 5.1 --- Manual curation of GenePRIMP results --- p.116 / Chapter 5.2 --- Functional annotation of local S. Typhimurium genomes --- p.118 / Chapter 5.3 --- Systemic infection studies --- p.120 / Chapter 5.4 --- Molecular markers identification and spiking experiments --- p.121 / Chapter 5.5 --- Conclusion and future perspectives --- p.122 / References --- p.124

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 71-80).

Diss. (sammanfattning) Stockholm : Univ., 2007. / Härtill 4 uppsatser.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 132-147).

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 56-75).

Thesis (Ph. D.) -- University of Texas Southwestern Medical Center at Dallas, 2005. / Vita. Bibliography: 85-95.

Ribonucleotide Reductases (RNRs) are crucial enzymes that catalyze reduction of ribonucleotides to deoxyribonucleotides, required for the biosynthesis of DNA. Vital role played by RNR in the biosynthesis of DNA and its control on cell growth made it one of the main targets for anticancer therapy. Several laboratories clarified the aspects of reaction cascades at active site of RNR. Biochemical studies of RNR by Stubbe for the inactivation of RDPR by 2'-chloro-2'-deoxyuridine-5'-diphosphate emphasizes departure of chlorine as an anion, while biomimetic studies by Robins with 6'-O-nitro-2'-chloro-homonucleosides emphasizes the elimination of chlorine substituent from 2'-position as a radical. To clarify the ambiguity in the mechanism of inhibition of RNR by 2'-chloro-2'-deoxyuridine, biomimetic reactions with model 6-O-nitro-1,5-dideoxyhomosugar derivatives were investigated. The study includes several modes: (i) synthesis of 6-O-nitro-1,5-dideoxyhomosugar derivatives with chlorine, bromine or tosyl substituent at the C2 position with ribo and arabino configurations, (ii) biomimetic studies of 6-O-nitro-1,5-dideoxyhomosugar derivatives with Bu3SnH/AIBN to provide chemical evidences to distinguish the nature of elimination of chlorine from 2'-chloro-2'-deoxyuridine upon its incubation with enzyme, and (iii) kinetic studies to differentiate between heterolytic or homolytic C2'-chlorine bond cleavage. In the second half of this dissertation, azido and sulfenamide modified nucleosides and 2-azidolyxofuranoside derivatives have been synthesized with the azido or sulfenamide substitution at a specific site in the sugar or in the base moiety. The electron-induced site specific formation of neutral aminyl radicals (RNH●) and their subsequent reactions have been investigated using ESR spectroscopy. In 2'-AZdC the RNH● site is attached to a 2o C-atom, where as in 4'-AZdC, the RNH● site is attached to a 3o C-atom, respectively. These studies elucidated how stereo and electronic environment affect formation and subsequent reactivity of various types of RNH● generated from azidonucleosides. To avoid the interaction of transient radical with nucleoside heterocyclic bases, 2-azidolyxofuranoside derivatives as a simpler abasic model were synthesized and studied with ESR spectroscopy. Aminyl radical generated from 2-azidolyxofuranoside derivatives subsequently abstracted hydrogen from C5 intramolecularly. These studies were designed to understand the mechanism of damage in various DNA model structures.

Ribonucleotide reductases (RNR) are essential enzymes that catalyze the reduction of ribonucleotides to 2'-deoxyribonucleotides, which is a critical step that produces precursors for DNA replication and repair. The inactivation of RNR, logically, would discontinue producing the precursors of the DNA of viral or cancer cells, which then would consequently end the cycle of DNA replication. Among different compounds that were found to be inhibitors of RNR, 2'-azido-2'-deoxynucleotide diphosphates (N3NDPs) have been investigated in depth as potent inhibitors of RNR. Decades of investigation has suggested that the inactivation of RNR by N3NDPs is a result of the formation of a nitrogen-centered radical (N•) that is covalently attached to the nucleotide at C3' and cysteine molecule C225 [3'-C(R-S-N•-C-OH)]. Biomimetic simulation reactions for the generation of the nitrogen-centered radicals similar to the one observed during the inactivation of the RNR by azionuclotides was investigated. The study included several modes: (i) theoretical calculation that showed the feasibility of the ring closure reaction between thiyl radicals and azido group; (ii) synthesis of the model azido nucleosides with a linker attached to C3' or C5' having a thiol or vicinal dithiol functionality; (iii) generation of the thiyl radical under both physiological and radiolysis conditions whose role is important in the initiation on RNR cascades; and (iv) analysis of the nitrogen-centered radical species formed during interaction between the thiyl radical and azido group by electron paramagnetic resonance spectroscopy (EPR). Characterization of the aminyl radical species formed during one electron attachment to the azido group of 2'-azido-2'-deoxyuridine and its stereospecifically labelled 1'-, 2'-, 3'-, 4'- or 5,6-[2H2]-analogues was also examined. This dissertation gave insight toward understanding the mechanism of the formation of the nitrogen-centered radical during the inactivation of RNRs by azidonucleotides as well as the mechanism of action of RNRs that might provide key information necessary for the development of the next generation of antiviral and anticancer drugs.