lac repressor: crystallization of intact tetramer and its complexes with inducer and operator DNA.

The intact lac repressor tetramer, which regulates expression of the lac operon in Escherichia coli, has been crystallized in the native form, with an inducer, and in a ternary complex with operator DNA and an anti-inducer. The crystals without DNA diffract to better than 3.5 A. They belong to the monoclinic space group C2 and have cell dimensions a = 164.7 A, b = 75.6 A, and c = 161.2 A, with alpha = gamma = 90 degrees and beta = 125.5 degrees. Cocrystals have been obtained with a number of different lac operator-related DNA fragments. The complex with a blunt-ended 16-base-pair strand yielded tetragonal bipyramids that diffract to 6.5 A. These protein-DNA cocrystals crack upon exposure to the gratuitous inducer isopropyl beta-D-thiogalactoside, suggesting a conformational change in the repressor-operator complex.