Differential mobility of the N-terminal headpiece in the lac-repressor protein.

It is shown by resolution enhancement and relaxation studies of the 360 MHz ¹H nuclear magnetic resonance spectra of the lac-repressor of Escherichia coli and the two fragments derived from it by limited tryptic digestion (the N-terminal headpiece and remaining T-core) that the majority of the relatively mobile residues in the intact lac-repressor are located in the headpiece. Although nuclear magnetic resonance data clearly indicate that the headpiece is a highly structured entity, even when isolated, it is a more mobile part of the repressor than the T-core.