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

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Abstract
The 360-MHz 1H NMR [PMR] spectrum of native [Escherichia coli] lac repressor headpiece (HP-51 or HP-59) contains a large number (> 30%) of aliphatic side-chain methyl and backbone .alpha.-CH resonances and 3 of 4 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 were made using resolution enhancement, double-resonance and difference spectra. There are 3 firmly assigned methyl resonances and 21 pairwise identifications of methyl resonances in HP-51. Comparison of HP-51 and HP-59 allows identification of 4 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.