THE DNA BEND ANGLE AND BINDING-AFFINITY OF AN HMG BOX INCREASED BY THE PRESENCE OF SHORT TERMINAL ARMS

The HMG box of human LEF-1 (hLEF-1, formerly TCF1 alpha) has been expr essed in four forms: a parent box of 81 amino acids and constructs hav ing either a 10 amino acid C-terminal extension, a 9 amino acid N-term inal extension, or both, These four species have been compared for DNA binding and bending ability using a 28 bp recognition sequence from t he TCR alpha-chain enhancer, In the bending assay, whereas the parent box and that with the N-terminal extension bent the DNA by 57/58 degre es, the box extended at the C-terminus bent the DNA by 77/78 degrees, irrespective of the presence or absence of the N-terminal extension, A 6-fold increase in DNA affinity also resulted from addition of both t erminal extensions, These observations redefine the functional boundar ies of the HMG box. The structure of a mouse LEF-1/DNA complex recentl y published [Love et al. (1995) Nature 376, 791-795] implies that the higher DNA affinity and in particular the increased bend angle observe d are consequences, at least in part, of the C-terminal extension span ning the major groove on the inside of the DNA bend.