Ion Concentration and Temperature Dependence of DNA Binding:  Comparison of PurR and LacI Repressor Proteins†

摘要: Purine repressor (PurR) binding to specific DNA is enhanced by complexing with purines, whereas lactose (LacI) diminished interaction inducer sugars despite 30% identity in their protein sequences and highly homologous tertiary structures. Nonetheless, switching from low- high-affinity binding, these proteins undergo a similar structural change which the hinge region connecting effector domains folds into an alpha-helix contacts minor groove. The differences response for should be manifest polyelectrolyte effect arises cations displaced positively charged side chains on quantitated measurement of affinity as function ion concentration. Consistent data proteins, operator PurR-purine complex involved approximately 15 pairs, value significantly greater than that corresponding state LacI (approximately 6 pairs). For both however, conversion low-affinity results decrease 2-fold number released per dimeric site. Heat capacity changes (DeltaC(p)) accompany derived buried apolar surface area, coupled folding, restriction motional freedom polar groups interface, also reflect between proteins. PurR-guanine accompanied DeltaC(p) (-2.8 kcal mol(-1) K(-1)) more negative observed previously (-0.9 -1.5 K(-1)), suggesting extensive folding and/or rigidity may occur upon PurR compared LacI. illustrate plasticity high-level sequence homology undermine efforts predict behavior basis such similarities.