Gal repressosome contains an antiparallel DNA loop

Gal repressosome assembly and repression of the gal operon in Escherichia c oli occurs when two dimeric GalR proteins and the histone-like HU protein b ind to cognate sites causing DNA looping. Structure-based genetic analysis defined the GalR surfaces interacting to form a stacked, V-shaped, tetramer ic structure. Stereochemical models of the four possible DNA loops compatib le with the GalR tetramer configuration were constructed using the sequence -dependent structural parameters of the interoperator DNA and conformation changes caused by GalR and asymmetric HU binding. Evaluation of their DNA e lastic energies gave unambiguous preference to a loop structure in which th e two gal operators adopt an antiparallel orientation causing undertwisting of DNA.