X-ray crystallography is one of the most powerful tools for the elucidation of the structure of biological macromolecules such as proteins and viruses. Crystallographic techniques are extensively used for investigations on protein structure, ligand-binding, mechanisms of enzyme catalyzed reactions, protein-protein interactions, role of metal ions in protein structure and function, structure of multi-enzyme complexes and viruses, protein dynamics and for a myriad other problems in structural biology. Crystallographic studies are essential for understanding the intricate details of the mechanism of action of enzymes at molecular level. Understanding the subtle differences between the pathogenic enzymes and host enzymes is necessary for the design of inhibitor molecules that specifically inhibit parasite enzymes. The current thesis deals with the application of biochemical and crystallographic techniques for understanding the structure and function of proteins from two pathogenic organisms – a plant virus Physalis Mottle Virus (PhMV), and a pathogenic bacterium, Salmonella typhimurium and also stress induced proteins from Oryza sativa. The thesis has been divided into seven chapters, with the first four chapters describing the work carried out on PhMV, while the rest of the chapters deal with the studies on stress response proteins from Oryza sativa and Salmonella typhimurium. The first part of the thesis deals with studies on viral capsids. Viruses are obligate parasites that have proteinaceous capsids enclosing the genetic material, which, in the case of small plant viruses, is invariably ss-RNA. X-ray diffraction studies on single crystals of viruses enable visualization of the structures of intact virus particles at near-atomic resolution. These studies provide detailed information regarding the coat protein folding, molecular interactions between protein subunits, flexibility of the N-and C-terminal segments and their probable importance in viral assembly, role of RNA in capsid assembly, nucleic acid (RNA)-protein interactions, the capsid structure and mechanism of assembly and disassembly. The present thesis deals with the capsid structure and analysis of the coat protein (CP) recombinant mutants of PhMV. Virus assembly, one of the important steps in the life cycle of a virus, involves specific interactions between the structural protein and cognate viral genome. This is a complex process that requires precise protein-protein and protein nucleic acid interactions. In fact, most of the biological functional units such as ribosomes and proteosomes also require highly co-ordinated macromolecular interactions for their functional expression. Viruses being simple in their architecture, serve as excellent model systems to understand mechanism of macromolecular assembly and provide necessary information for the development of antiviral therapeutics, especially in animal viruses. PhMV is a plant virus infecting several members of Solanaceae family. It belongs to the tymoviridae group of single stranded RNA viruses. Its genome is encapsidated in a shell comprising of 180 (architecture based on T = 3 icosahedral lattice) chemically identical coat protein (CP) subunits (~ 20,000Da) arranged with icosahedral symmetry. In an earlier phase of work, PhMV purified from infected plant leaves was crystallized in the space group R3 (a = 294.56 Å,  = 59.86). X-ray diffraction data to 3.8 Å resolution were recorded on films by screenless oscillation photography. Using this data of severely limited quality and poor completion (40%), the structure PhMV was determined by molecular replacement using the related turnip yellow mosaic virus (TYMV) structure as the phasing model. There was therefore a need to re-determine and improve the structure, which could be useful for understanding the earlier detailed studies on its biophysical properties. As a continuation of these studies, the present investigations were conceived with the goal of determining the natural top and bottom component capsid structures of PhMV. Investigations were also carried out to examine the possibility of enhancing the diffraction quality of PhMV crystals. The thesis begins with a review of the current literature on the available crystal structures of viruses and their implications for capsid assembly (chapter I). All experimental and computational methods used during the course of investigations are described in chapter II, as most of these are applicable to all the structure determinations and analyses. The experimental procedures described include cloning, overexpression, purification, crystallization and intensity data collection. Computational methods covered include details of various programs used during data processing, structure solution, refinement, model building, validation and analysis. Chapter III describes structural studies on top and bottom components of PhMV. Purified tymoviruses including PhMV are found to contain two classes of particles that sediment at different velocities through sucrose gradients and are called the top (sedimentation coefficient 54 Svedberg units(S)) and the bottom (115S) components. The top component particles are either devoid of RNA or contain only a small subgenomic RNA (5%) while the bottom component particles contain the full length genomic RNA. Only the bottom component is infectious. The top and bottom components were separately crystallized in P1 and R3 space groups, respectively. It is of interest to note that crystals of the bottom component obtained earlier belonged to R3 space group while recombinant capsids that lack of full length RNA as in natural top component crystallized in the P1 space group. A polyalanine model of the homologous TYMV was used as the phasing model to determine the structures of these particles by molecular replacement using the program AMoRe. The refinement of top and bottom component capsid structures were carried out using CNS version 1.1 and the polypeptide models were built into the final electron-density map using the interactive graphics program O. The quality of the map was sufficient for building the model and unambiguous positioning of the side chains. There is a significant difference in the radius of the top and bottom component capsids, the top component being 5 Å larger in radius. Thus, RNA makes the capsid more compact, even though RNA is not a pre-requisite for capsid assembly. Partially ordered RNA was observed in the bottom component. The refined models could form the basis for understanding the architecture, protein-protein interactions, protein-nucleic acid interactions, stability and assembly of PhMV. Chapter IV provides a detailed description of the mutations carried out on PhMV coat protein towards enhancing the diffraction quality of crystals. The gene coding for PhMV coat protein (PhMVCP) and several of its deletion and substitution mutants were originally cloned in pRSETC and pET-21 vectors by Mira Sastri and Uma Shankar in Prof. Savithri’s laboratory at the Department of Biochemistry. It was observed that the recombinant intact coat protein and several mutants lacking up to 30 amino acids from the N-terminal end could assemble into empty shells resembling the natural top component. None of these deletion mutants crystallized in forms that diffracted to high resolution. Based on the intersubunit contacts observed, three more site-specific mutants were designed. These three mutants were expressed in BL21 (DE3), purified and crystallized. Even these mutant crystals did not diffract to high resolution. The polypeptide fold of PhMV coat protein therefore was carefully examined for probable reasons. It was found that PhMV subunit has three major cavities. Three cavities are likely to increase the flexibility of protein subunits, which in turn may result in crystals of poor quality. Mutations V52W, S158Q and A160L were shown to fill up these cavities and with the view of obtaining better crystals. These site specific mutations were carried out the mutant proteins were purified. It was shown that the recombinant capsids are stable and possess T=3 architecture. Two mutants were crystallized and a data set for V52W extending to 6.0 Å resolution could be collected. Due to the limited resolution, further work was not pursued. It is plausible that the triple mutant will diffract to higher resolution. The second part of the thesis deals with stress response proteins from Oryza sativa and Salmonella typhimurium. It is known that viral infection and abiotic and biotic stresses induce a network of proteins in plants. Chapter V presents a review of the current literature on stress proteins, focusing mainly on Oryza sativa and S. typhimurium stress response proteins. Chapter VI describes the over expression of stress proteins SAP1 and SAP2 from rice. These stress related proteins confer tolerance to cold, dehydration and salt stress in rice. These proteins have been cloned in the expression vector pEt-28(a) and expressed in E. coli strain BL21 CodonPlus(DE3)RIL. The proteins were purified and crystallization trials were made. However, there were no hits. In an attempt to get crystals, nine deletion constructs of SAP1 were designed eliminating potentially disordered and unfolded regions based on a bioinformatics analysis. Crystallization trails are being carried out on three of the constructs. Structural studies on a universal stress protein from Salmonella typhimurium, which shares homology with the rice universal stress proteins, was initiated. Apart from this, several other stress related proteins of Salmonella typhimurium have also been selected for structural and functional studies. These include YdaA, YbdQ, Yic, Ynaf, Yec, Spy and Usb. All these were cloned and expressed in E. coli. Out of seven proteins, Ynaf, YdaA and YbdQ were found in the soluble fraction and were expressed in quantities suitable for structural studies. I could crystallize YdaA and Ynaf. X-ray diffraction data to resolutions of 3.6 Å and 2.3 Å were collected on crystals of YdaA and YnaF, respectively. A tentative structure of YnaF has been obtained. Further attempts to determine these structures are in progress. Biophysical, Biochemical functional characterization of YdaA and YnaF proteins are described. Structural studies on mannose-6-phosphate isomerase, an enzyme related to stress regulatory proteins from S. typhimurium are dealt with in Chapter VII. Mannose 6-phosphate isomerase (MPI) catalyzes the interconversion of mannose 6-phosphate and fructose 6-phosphate. The structure could be solved in its apo and holo forms (with two different metal atoms, Y3+ and Zn2+), and complexed with the cyclic form of the substrate fructose 6-phosphate (F6P) and Zn2+. Isomerization involves acid/base catalysis with proton transfer between C1 and C2 atoms of the substrate. Lys 132, His 131, His 99 and Asp 270 are close to the substrate and are likely to be the residues involved in proton transfer. Interactions observed at the active site suggest that the ring opening step is catalyzed by His 99 and Asp 270. An active site loop consisting of residues 130-133 undergoes conformational changes upon substrate binding. The metal ion is not close to the substrate atoms involved in proton transfer. Binding of the metal induces structural order in the loop consisting of residues 50-54. Hence, the metal atom does not appear to play a direct role in catalysis, but is probably important for maintaining the architecture of the active site. Based on these structures and earlier biochemical work, a probable isomerization mechanism has been proposed. The thesis concludes with a brief discussion on the future prospects of the work. The following manuscripts have been published or will be communicated for publication based on the results presented in the thesis:

Chapter Introduction Salmonellae are gram negative bacteria that cause gastroenteritis and entericfever. S. enterica is divided into seven phylogenetic groups, subspecies 1, 2,3a, 3b, and 4, 6, 7. Subspecies1 includes 1,367 serovars, some of which are commonly isolated from infected birds and mammals. The other subspecies mainly colonize cold blooded animals. Salmonella typhimurium, Salmonella typhiandSalmonella enteritidis are some of the serovars, which belong to s.enterica species. S. typhimurium is one of the important causes for food poisoning in humans. It causes typhoid like fever in mice. In immuno compromised patients the infection is often fatal if it is not treated with antibiotics. Clinical features of food poisoning include abdominal pain, vomiting, nausea, abdominal cramps, dehydration etc. S. typhi causes typhoid fever in humans. No other host has been identified for this serovar. Main source of infection is contaminated food and water. No age is exempted but it is less common before2 years. Incubation period is 360 days. Clinical features include stepladder type fever, malaise, headache, hepato splenomegaly, coated tongue, Neutrogena etc. It may be fatal if untreated. Among the serovars of Salmonella infecting humans S. typhimurium and S. typhi are the most important. While S. typhimurium infects many host species including birds and mammals, S. typhi is single host adapted and infects only human. The single host adaptation of S. typhi presents it with the need for establishing are servoir of infection in the community which can serve as a source of fresh infection. Also the single host adaptation of S. typhi has made it a highly specialized pathogen which has evolved certain unique genes needed for human colonization at the same time has lost a set of genes which are needed for survival in other hosts and in the highly variable external environment. This has led to the accumulation of a vast number of pseudo genesin S. Typhi. A comparative study of the two serovars is useful in many ways. Due to varied host defense systems encountered by the two serovars owing to different niche of infection the bacterial counter defense mechanisms are also different. By focusing on the differences between genes involved in the bacterial defense of host immune response we can decipher the role played by various genes in combating the antibacterial host response. Chapter 2 The role of TolA and peptidoglycan modification in detergent resistance of pathogenic Salmonella The major Salmonella serovars that infect human are Salmonella enterica serovar Typhi (S.typhi) which cause systemic typhoid and Salmonella enterica serovar Typhimurium (S. typhimurium) which cause gastro enteritis. S. typhi resides in the gall bladder during chronic infection and S .typhimurium infects intestine .Thus both pathogens encounter high concentrations of bile and have developed mechanisms to counter it. The Tol Pal complex spanning the outermembrane and the inner cytoplasmic membrane plays an important role in maintaining the stability of the outer membrane and providing detergent resistance. The tolA gene of S. Typhi Is shorter by 27 aminoacid than S. typhimurium. The tolA gene knockout of S. typhimurium and S. typhi differed in their tritonX resistance behavoiur, morphology and low osmolality tolerance. S. typhi tolA was unable to complement the tolA defect in S. typhimurium which could probably due to the difference in the peptidoglycan layer. An analys is of the peptidoglycan modifying genes of both the serovars revealed that dacD, pbgP, ynhG are different. dacD, pbgP genes are pseudogenes in S. typhi and ynhG has a major deletion in S. typhi. Further studies reveal that a double knockout of dacD and pbpG in S. typhimurium makes it sensitive to low osmolality similar to S. typhi. Based on these results we propose a mechanism, where shortening of TolA increases detergent resistance by bringing the outer membrane into closer contact with the peptidoglycan layer, but this is achieved at the cost of reduced Lpp (Bruan’slipoprotein) peptidoglycan linkage which plays a major role in low osmolality tolerance. The pathogen S. typhi is highly adapted to the human host and cannot infect any other host. The single host adaptation and the need to survive in high concentrations of bile have made S. typhi to acquire higher bile resistance at the cost of lowered osmotic tolerance through shortening TolA and reduced Lpp and peptidoglycan binding. Chapter 3 Development of a DNA vaccine against Salmonella The immune response against Salmonella is multifaceted involving both the innate and the adaptive immune system. The characterization of specific Salmonella antigens inducing immune response could critically contribute to the development of epitope based vaccines for Salmonella. We have tried to identify aprotective Tcellepitope (s) of Salmonella, as cell mediated immunity conferred by CD8+T cells is the most crucial subset conferring protective immunity against Salmonella. It being a proven fact that secreted proteins are better in inducing cell mediated immunity than cell surface and cytosolic antigens, we have analyzed all the GenBank annotated Salmonella pathogenicity island 1 and 2 secreted proteins of S. typhimurium and S. typhi. They were subjected to BIMAS and SYFPEITHI analysis to map MHCI and MHC II binding epitopes. The huge profile of possible T cell epitopes obtained from the two classes of secreted proteins were tabulated and using a scoring system that considers the binding affinity and promiscuity of binding to more than one allele, SopB and SifB were chosen for experimental confirmation in murine immunization model. The entire Sop Band SifB genes were cloned into DNA vaccine vectors and were administered along with live attenuated Salmonella and it was found that SopB vaccination reduced the bacterial burden of organs by about 5fold on day4 and day8 after challenge with virulent Salmonella and proved to be a more efficient vaccination strategy than live attenuated bacteria alone. Chapter 4 PCR based diagnosis and Serovar Determination of Blood Borne Salmonella Typhoid fever is becoming an ever increasing threat in the developing countries. We have improved considerably upon the existing PCR based diagnosis method by designing primers against a region which is unique to S. typhiand S. paratyphiA, corresponding to the gene STY0312 in S. typhi and its homolog SPA2476 in S. paratyphiA. An additional set of primers amplify another region in S. typhi CT18 and S. typhiTy2 corresponding to the region between the genes STY0313 toSTY0316 but which is absent in S.paratyphi A. The threat of false negative result arising due to mutation in hypervariable genes has been reduced by targeting a gene unique to typhoidal Salmonella as a diagnostic marker. The amplified region has been tested for genomic stability by amplifying them from clinical is olates of patients from various geographical locations in India, there by showing that this region is potentially stable. These set of primers can also differentiate between S. typhiCT18, S. typhiTy2 and S. paratyphi A which have stable deletions in this specific locus. The PCR assay designed in this study has a sensitivityof95%ascompared to the Widal test which had only 63%. As observed, in certain cases the PCR assay was more sensitive than the blood culture test as the PCR based detection could also detect dead bacteria.