CRYSTAL-STRUCTURE OF LACI MEMBER, PURR, BOUND TO DNA - MINOR-GROOVE BINDING BY ALPHA-HELICES

The three-dimensional structure of a ternary complex of the purine rep ressor, PurR, bound to both its corepressor, hypoxanthine, and the 16- base pair purF operator site has been solved at 2.7 Angstrom resolutio n by x-ray crystallography. The bipartite Structure of PurR consists o f an amino-terminal DNA-binding domain and a larger carboxyl-terminal corepressor binding and dimerization domain that is similar to that of the bacterial periplasmic binding proteins. The DNA-binding domain co ntains alpha helix-turn-helix motif that makes base-specific contacts in the major groove of the DNA. Base contacts are also made by residue s of symmetry-related a helices, the ''hinge'' helices, which bind dee ply in the minor groove, Critical to hinge helix-minor groove binding is the intercalation of the side chains of Leu(54) and its symmetry-re lated mate, Leu(54'), into the central CpG-base pair step. These resid ues thereby act as ''leucine levers'' to pry open the minor groove and kink the purF operator by 45 degrees.