NMR Study of the Interaction between the lac Repressor and the lac Operator

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Abstract
Binding of the lac repressor headpiece, the N-terminal region of the lac repressor, to the lac operator of Escherichia coli was studied by 1H-NMR spectroscopy. Two DNA fragments, of 51 base pairs and 62 base pairs, containing the lac operator region, were investigated. The signals of their H-bonded imino protons were well resolved in the 500-MHz NMR spectra. The spectra of the free lac operator DNA are similar to those obtained from ring-current-shift calculations for a B-DNA structure. Complex formation with the headpiece led to small but characteristic changes in the spectra. The fact that very few imino resonances shifted upon addition of headpiece and the variety in direction and size of these chemical shifts indicate the formation of a specific complex between the lac repressor and the lac operator. The observed changes in the resonance positions exclude the intercalation of tyrosine residues of the headpiece between adjacent base pairs of the lac operator and the formation of a cruciform structure. They rather reflect a small conformational transition in the DNA itself, caused for example by an alteration in the tilt of a few base pairs or a shift of the keto-enol tautomeric equilibrium of the bases towards the enolic form.