THE SOLUTION STRUCTURE OF AN HMG-I(Y)-DNA COMPLEX DEFINES A NEW ARCHITECTURAL MINOR-GROOVE BINDING MOTIF

The solution structure of a complex between a truncated form of HMG-I( Y), consisting of the second and third DNA binding domains (residues 5 1-90), and a DNA dodecamer containing the PRDII site of the interferon -beta promoter has been solved by multidimensional nuclear magnetic re sonance spectroscopy. The stoichiometry of the complex is one molecule of HMC-I(Y) to two molecules of DNA. The structure reveals a new arch itectural minor groove binding motif which stabilizes B-DNA, thereby f acilitating the binding of other transcription factors in the opposing major groove. The interactions involve a central Arg-Gly-Arg motif to gether with two other modules that participate in extensive hydrophobi c and polar contacts. The absence of one of these modules in the third DNA binding domain accounts for its similar to 100 fold reduced affin ity relative to the second one.