Lac repressor and its tryptic core have been investigated by electro-optical methods. The reduced dichroism measured as a function of the electric field strength is not consistent with an induced dipole, but indicates the existence of a strong permanent dipole moment ( ∼ 4 × 10-27C m) for the holo-repressor, which is almost independent of ion concentration and pH. A dominant contribution of a permanent dipole is also demonstrated by the shape of the dichroism rise curve. The experimental data are not consistent with a counterion polarization phenomenon and also do not indicate a major contribution from proton fluctuations. Probably the nature of the dipole is similar to that found for compounds with a tetrahedral substitution by angular residues. Other potential models involve large conformational fluctuations or inherent asymmetry of the lac repressor. Rotation time constants obtained from the dichroism decay are not consistent with a spherical shape, for either the holo- and or core repressor. A simple interpretation of the data by prolate ellipsoids suggests a short diameter of 6 nm for both holo- and core repressor and long diameters of 14 and 12 nm for holo- and core repressor, respectively. Addition of the inducer isopropyl-β-D-thiogalactopyranoside leads to a change of the limit dichroism, but does not affect the rotation time constants within experimental accuracy. © 1987.
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