High-resolution nuclear magnetic resonance studies of the Lac repressor. 2. Partial analysis of the aliphatic region of the Lac repressor headpiece spectrum.

The 360-MHz 1H NMR spectrum of native lac repressor headpiece (HP-51 or HP-59) contains a large number (greater than 30%) of aliphatic side-chain methyl and backbone alpha-CH resonances and three of four aromatic tyrosine multiplet resonances shifted to high-field chemical shift positions, indicating the presence of extensive folded structure. Denaturation leads to loss of the NMR chemical shift differences. Resonance identifications of the 27 methyl-possessing amino acids in HP-59 have been made by using resolution enhancement, double-resonance, and difference spectra. There are three firmly assigned methyl resonances and 21 pairwise identifications of methyl resonances in HP-51. Comparison of HP-51 and HP-59 allows identification of four additional methyl groups in amino acid residues 52--59. The sequence HP-50--59 is not essential to maintain the structure of HP-59, but it is of interest itself as the flexible hinge portion connecting HP to the tetrameric core of whole repressor.