The ATP-PRTase enzyme catalyses the first committed step of histidine biosynthesis in archaea, bacteria, fungi and plants.1 As the catalyst of an energetically expensive pathway, ATP-PRTase is subject to a sophisticated, multilevel regulatory system.2 There are two families of this enzyme, the long form (HisGL) and the short form (HisGS) that differ in their molecular architecture. A single HisGL chain comprises three domains. Domains I and II house the active site of HisGL while domain III, a regulatory domain, forms the binding site for histidine as an allosteric inhibitor. The long form ATP-PRTase adopts a homo-hexameric quaternary structure.3,4 HisGS comprises a similar catalytic core to HisGL but is devoid of the regulatory domain and associates with a second protein, HisZ, to form a hetero-octameric assembly.5 This thesis explores the allosteric regulation of the short form ATP-PRTase, as well as the functional and evolutionary relationship between the two families. New insight into the mode allosteric inhibition of the short form ATP-PRTase from Lactococcus lactis is reported in chapter two. A conformational change upon histidine binding was revealed by small angle X-ray scattering, illuminating a potential mechanism for the allosteric inhibition of the enzyme. Additionally, characterisation of histidine binding to HisZ by isothermal titration calorimetry, in the presence and absence of HisGS, provided evidence toward the location of the functional allosteric binding site within the HisZ subunit. Chapter three details the extensive effort towards the purification of the short form ATP-PRTase from Neisseria menigitidis, the causative agent of bacterial meningitis. This enzyme is of particular interest as a potential target for novel, potent inhibitors to combat this disease. The attempts to purify the long form ATP-PRTase from E. coli, in order to clarify earlier research on the functional multimeric state of the enzyme, are also discussed. Chapter four reports the investigation of a third ATP-PRTase sequence architecture, in which hisZ and hisGS comprise a single open reading frame, forming a putative fusion enzyme. The engineering of two covalent linkers between HisZ and HisGS from L. lactis and the transfer of the regulatory domain from HisGL to HisGS, is also discussed, in an attempt to delineate the evolutionary pathway of the ATP-PRTase enzymes. Finally, the in vivo activity of each functional and putative ATP-PRTase was assessed by E. coli BW25113∆hisG complementation assays.