Members of the Leucine-Responsive regulatory protein (Lrp)/Asparagine synthase C protein (AsnC) family are global or specific transcriptional regulators widely present in prokaryotes (both in bacteria and archaea) and play a vital role in amino acid metabolism, transport, and other related biological processes [1], [2], [3]. The best-characterized member is Escherichia coli Lrp, which regulates multiple genes and operons, including some of those involved in amino acid metabolism [4], [5]. Depending on the binding of exogenous Leucine to the Lrp, metabolism switches between famine and feast modes. Therefore, transcription factors of this kind are referred to as Feast/Famine Regulatory Protein (FFRP) [5], [6] an approximate molecular weight of 15 kDa and are mainly comprised of two domains: The N-terminal Helix-Turn-Helix (HTH) DNA binding domain and the C-terminal effector binding (or regulatory) domain, which is linked through a flexible hinge region [7]. The regulatory domain, also referred to as the stand-alone Regulation of Amino Acid Metabolism (RAM) domain, consists of an αβ-sandwich (βαββαβ-fold). RAM domain is strikingly similar to Aspartokinase, Chrismate mutase, and TyrA (ACT) domains, which are ubiquitous small-molecule-binding and allosteric regulatory domains in many metabolic enzymes [8], [9]. The RAM domain contains Type-I and Type-II active site pockets. The binding of exogenous amino acids in the Type-II binding site regulates the transcriptional mechanism, while binding in the Type-I site allosterically downregulates the transcription [10]. For example, the binding of tryptophan to the effector binding domain of the tryptophan responsive regulatory (TRP) protein modulates the binding affinity of protein-DNA complex, and thereby the transcription is preferred to be either an activation or a disturbance [2]. Proteins of the Lrp/AsnC family form higher-order oligomers in solution. This oligomer assembly of the protein family is favorably determined by the binding of exogenous amino acids to the apo-protein [2], [11]. P. furiosus LrpA, E. coli AsnC, B. subtilis LrpC, and M. tuberculosis H37Rv (Rv2779c) form dimer, octamer, dodecamer, and decamer assemblies, respectively [12], [13]. Several FFRP crystal structures have been determined with exogenous amino acids and DNA molecule bound complexes. A crystal structure of exogenous amino acid, which induced the open octameric assembly of FFRPs (PDB ID: 2GQQ), was also reported earlier [14]. FFRPs commonly bind in the duplex promoter regions TTTT or AAAA for transcriptional regulation and better binding affinity, while changing any one of those bases weakens the binding affinity or molecular interactions of the complex [6]. Despite the extensive collection of the protein structures, the molecular mechanism, oligomeric association, and conformational changes induced by the binding of exogenous amino acids are largely unknown in the family of FFRPs. We performed extensive computational analysis on PH0140 using docking and MD simulations. The results strongly suggest that the tryptophan amino acid exhibits significantly higher docking scores and regulates transcription through allosteric network communication [2]. Therefore, we sought to experimentally validate the binding of tryptophan and transcriptional regulation of PH0140. In this study, we elucidate the structural fold, binding specificity, oligomeric association, and conformation changes necessary for the transcription of PH0140 (FFRPs) from Pyrococcus horikoshii OT3 using X-ray crystallography, biophysical and molecular modeling studies.