The secondary structure of the ligand response domain of the Haemophilus influenzae tyrosine repressor, TyrR(1rd), was investigated using CD spectroscopy which revealed 42.5% alpha -helix, 17.6% beta -sheet, and 39.9% loops. Quaternary structure analysis by fluorescence anisotropy showed that TyrR(Ird) is monomeric at a concentration of 100 nM to 2 muM but that the protein readily dimerizes in the presence of its natural ligand ATP. Equilibrium unfolding studies of TyrR(1rd) using guanidinium hydrochloride suggested a two-state model with no detectable stable intermediates. The unfolding transition monitored by CD spectroscopy was responsive to tyrosine and ATP resulting in a shift to higher denaturant concentrations in the presence of these ligands. Differential scanning calorimetry yielded melting temperatures, T-m, of 51.15 and 58.07 degreesC for the unliganded and for the ATP-liganded protein, respectively. ATP is thus proposed to be a major structural cofactor for the molecular architecture of TyrR(1rd)