High-resolution nuclear magnetic resonance studies of the Lac repressor. 3. Unfolding of the Lac repressor headpiece.

At temperatures below 20 degrees C, the lac repressor headpiece (N-terminal amino acids 1--51) has a well-defined structure which is independent of ionic strength. Its unfolding with increasing temperature proceeds gradually with a characteristic transition temperature which depends on ionic strength. Unfolding has been studied by using NMR and CD. Shifts of several methyl and all of the tyrosyl resonances can be followed, allowing a detailed analysis of the temperature denaturation. At high ionic strength (1 M), the unfolding is complete at 85 degrees C, while at low ionic strength (0.01 M), it is complete by 65 degrees C. Native and partially unfolded structures are in rapid exchange during the unfolding, and the process appears completely reversible at all ionic strengths.