X-ray crystallography is a powerful method for determining the three-dimensional structures of biological macromolecules at atomic resolution. Crystallography can reliably provide the answer to many structure related questions, from global folds to atomic details of bonding. Crystallographic techniques find wide applications in understanding macromolecular assembly, enzyme mechanism, mode of activation of enzymes, substrate-specificity, ligand-binding properties, domain movement etc. The knowledge of accurate molecular structures is also a prerequisite for rational drug design and for structure based functional studies to aid the development of effective therapeutic agents. The current thesis can be broadly divided into two major parts. The first four chapters deal with assembly studies that have been carried out on Sesbania mosaic virus and the next two chapters describe the structure and function of a stationary phase survival protein, SurE from Salmonella typhimurium. In both studies X-ray crystallographic techniques have been used extensively for the structural studies. Viruses are obligate parasites with a proteinaceous capsid enclosing the genetic material. For genetic economy, several copies of capsid proteins self assemble to form complex virus capsids. Due to their intricate symmetric structures, viruses are considered as minute marvels of molecular architecture and study of virus structures serve as a paradigm for solutions to problems concerning macromolecular assembly and function in general. Crystallography provides a means of visualizing intact virus particles as well as their isolated constituent proteins and enzymes at near-atomic resolution, and is thus an extraordinarily powerful tool for understanding the function of these biological systems. Protein-protein interactions, protein-nucleic acid interactions, metal-ion mediated interactions, interactions between capsid proteins and auxillary or scaffolding proteins and particle maturation or post processing of capsid protein subunits are various elements that play a role in capsid assembly. Many structural and sequential motifs have been proposed as important conformational switches of capsid assembly. A functional analysis of these motifs by way of mutations in the capsid protein and structural studies of these mutants can provide further insight into capsid assembly pathways. Interaction between capsid protein subunits can determine the size and robustness of the capsid. Analysis of protein-protein interactions can help in understanding the principles of self-assembly. Arresting capsid assembly by disrupting intersubunit interactions and trapping the assembly intermediates will be helpful to delineate the changes that happen in capsid protein during the course of assembly and understand assembly pathways. Sesbania mosaic virus (SeMV) is a plant virus with a positive sense single-stranded RNA genome and belongs to the Sobemovirus genus. The protein and nucleic acids of SeMV can be separated and reassembled in vitro. Also, expression of the coat protein (CP) gene of SeMV in E. coli leads to the formation of virus like particles (VLPs). Therefore, SeMV is an excellent model system to study the assembly pathways that lead to the formation of complex virus shells. Earlier structural and functional studies on the native virus and the recombinant capsid protein and its various mutants have revealed the following: SeMV is a T=3 virus with chemically identical A-, B- and C-subunits occupying quasi equivalent positions in the icosahedral asymmetric unit of the virus particle. The A-type subunits form pentamers at the five-fold, and the B- and C- type subunits form hexamers at the icosahedral three-fold axes. The amino terminus of the polypeptide is ordered from residue 72 in the A- and B- subunits whereas it is ordered from residue 44 in the C-subunit. The disordered segment in all the subunits has an arginine rich motif (N-ARM). The segment ordered only in C-subunits has a -annulus structure that promotes intersubunit interactions at the quasi six-fold and a -segment (A). The virus is stabilized by protein-protein, protein–RNA and Ca2+ mediated protein-protein interactions. Virus like particles (VLPs) formed by the expression of full length CP encapsidate 23 S E. coli rRNA and CP mRNA. Expression of a deletion mutant lacking the N-terminal 65 residues (rCP∆N65) which results in the removal of the N-ARM, the -annulus and the A leads to the formation of stable T=1 particles. The -annulus, which was earlier believed to be an important molecular switch controlling the assembly of T=3 VLPs was found to be dispensable. The N-ARM, though important for RNA encapsidation, was not essential for capsid assembly . Depletion of Ca2+ ions led to slight swelling of virus particles and significantly reduced stability. Extensive studies on the VLPs suggested that the assembly is most likely initiated by the dimers of the capsid protein. Following a brief account of the historical highlights in the field of structural virology, a review of current literature on the available crystal structures of viruses and various assembly studies on viruses that have been carried out with emphasis on role of nucleic acid mediated interactions, protein-protein interactions and role of specific residues and ion-mediated interactions in assembly are presented in Chapter I of the thesis. A separate section in this chapter deals with the disassembly experiments that have led to the formation of smaller oligomers of spherical viruses. This chapter also gives an account of the earlier work that has been carried out on SeMV, which is the model system of study for the present thesis. Chapter II describes in detail the structural studies on the β-annulus deletion mutant of SeMV. A unique feature of several T = 3 icosahedral viruses is the presence of a structure called the β-annulus formed by extensive hydrogen bonding between protein subunits related by icosahedral three-fold axis of symmetry. This unique structure has been suggested as a molecular switch that determines the T = 3 capsid assembly. In order to examine the importance of the β-annulus, a deletion mutant of Sesbania mosaic virus coat protein in which residues 48–59 involved in the formation of the β-annulus were deleted retaining the rest of the residues in the amino terminal segment (rCP (Δ48–59)) was constructed. When expressed in Escherichia coli, the mutant protein assembled into virus like particles of size close to that of the wild type virus particles. The purified capsids were crystallized and their three dimensional structure was determined at 3.6Å resolution by X-ray crystallography. The mutant capsid structure closely resembled that of the native virus particles. However, surprisingly, the structure revealed that the assembly of the particles has proceeded without the formation of the β-annulus. Therefore, the β-annulus is not essential for T = 3 capsid assembly as speculated earlier and may be formed as a consequence of the particle assembly. This is the first structural demonstration that the virus particle morphology with and without the β-annulus could be closely similar. Chapter III begins with a detailed description of the interfacial residue mutations that have been carried out in SeMV with the aim of disrupting assembly and trapping an assembly intermediate. These mutations were performed in rCP as well as rCP∆N65 gene. Among these, a single point mutation of a Trp 170 to a charged residue (either Glu or Lys) arrested virus assembly and resulted in stable dimers of the capsid protein. The chapter also gives an account of the biophysical characterization of these mutants. rCP∆N65 dimer mutants showed a characteristic 230 nm peak in CD spectral studies which may be due to the interactions of a stretch of aromatic residues in the capsid protein. The isolated dimers were more susceptible to trypsin cleavage compared to the assembled capsids due to the exposed basic amino terminus. Thermal melting studies showed that the isolated dimer mutants were much less stable when compared to the assembled capsids, probably due to the loss of intersubunit interactions and Ca2+ mediated interactions. The structure of one of the isolated dimer mutant- rCP∆N65W170K was solved to a resolution of 2.65Å. Chapter IV describes the crystal structure analysis of the rCP∆N65W170K mutant dimer and compares its structure with the dimers of native virus, T=3 and T=1 VLPs. A number of structural changes occur especially in the loop and interfacial regions during the course of assembly. The dimer in solution was “more relaxed” than the dimer that initiates assembly. Ca2+ ion is not bound and consequently the C-terminal residues are disordered. The FG loop, which interacts with RNA, was found to be flexible and adopts a different conformation in the unassembled dimer. The present thesis also deals with the structural and functional studies of a phosphatase, SurE, the stationary phase survival protein from Salmonella typhimurium. Chapter V provides a general introduction on Salmonella, which is a mesophilic food borne pathogen, its general features, classification and stress responses. This chapter also gives an account of stationary phase in bacteria and stress responses. A brief description about phosphatases and their classification is also presented in this chapter. Following this, a review of the current literature on the structural, biochemical and functional role of stress related proteins and phylogenetic and enzymatic studies of various homologues of SurE are described in detail. Chapter VI deals with the detailed crystal structure analysis of SurE, the first stationary phase survival protein from a mesophilic organism. SurE, of Salmonella typhimurium forms part of a stress survival operon regulated by the stationary phase RNA polymerase alternative sigma factor. SurE is known to improve bacterial viability during stress conditions. It functions as a phosphatase specific to nucleoside monophosphates. Here we report the X-ray crystal structure of SurE from Salmonella typhimurium (St SurE). The protein crystallized in two forms- orthorhombic F222 and monoclinic C2. The two structures were determined to resolutions of 1.7Å and 2.7Å, respectively. The protein exists as a domain swapped dimer. The residue Asp 230 is involved in several interactions that are probably crucial for domain swapping. A divalent metal ion is found at the active site of the enzyme, which is consistent with the divalent metal-ion dependent activity of the enzyme. Interactions of the conserved DD motif present at the N-terminus with the phosphate and the Mg2+ present in the active site suggest that these residues play an important role in enzyme activity. The divalent metal ion specificity and the kinetic constants of SurE were determined using the generic phosphatase substrate- para- Nitro Phenyl Phosphate. The enzyme was inactive in the absence of divalent cations and was most active in the presence of Mg2+. Thermal denaturation studies showed that St SurE is much less stable compared to its homologues and an attempt was made to understand the molecular basis of the lower thermal stability based on solvation free energy. The thesis concludes with a brief summary of the entire work that have been presented and future prospects. The various crystallographic, biochemical and biophysical techniques employed in the investigations are described under the section experimental techniques in Appendix I and the NCS matrices used in the structure solution of the β-annulus deletion mutant are listed in Appendix II.