THE DNA-SEQUENCE SPECIFICITY OF HMG BOXES LIES IN THE MINOR WING OF THE STRUCTURE

To establish the basis of sequence-specific DNA recognition by HMG box es we separately transferred the minor and major wings from the sequen ce-specific HMG box of TCF1 alpha into their equivalent position in th e non-sequence-specific box 2 of HMG1. Thus chimera THT1 contains the minor wing (of 11 N-terminal and 25 C-terminal residues) from the HMG box of TCF1 alpha and the major wing (the 45 residue central section) from HMG1 box 2, whilst the situation is reversed in chimera HTH1. The structural integrity of the two chimeric proteins was established by CD, NMR and their binding to four-way junction DNA, Gel retardation an d circular permutation assays showed that only chimera THT1, containin g the TCF1 alpha minor wing, formed a sequence-specific complex and be nt the DNA. The bend angle was estimated to be 59 degrees for chimera THT1 and 52 degrees for the HMG box of TCF1 alpha. Our results, in com bination with mutagenesis and other data, suggests a model for the DNA binding of HMG boxes in which the N-terminal residues and part of hel ix 1 contact the minor groove on the outside of a bent DNA duplex.