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A complex between the lac repressor headpiece and a fully symmetric
tight-binding 22 bp lac operator was studied by 2D NMR. Several 2D NOE spectra
were recorded for the complex in both H2O and 2H2O. Many NOE cross-peaks between
the headpiece and DNA could be identified, and changes in the chemical shift of
the DNA protons upon complex formation were analyzed. Comparison of these data
with those obtained for a complex between the headpiece and a 14 bp
half-operator, studied previously [Boelens, R., Scheek, R. M., Lamerichs, R. M.
J. N., de Vlieg, J., van Boom, J. H., & Kaptein, R. (1987) in DNA-ligand
interactions (Guschlbauer, W., & Saenger, W., Eds.) pp 191-215, Plenum, New
York], shows that two headpieces form a specific complex with the 22 bp lac
operator in which each headpiece binds in the same way as found for the 14 bp
complex. The orientation of the recognition helix in the major groove of DNA in
these complexes is opposite with respect to the dyad axis to that found for
other repressors.
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