DNA bending induced by high mobility group proteins studied by fluorescence resonance energy transfer

The HMG domains of the chromosomal high mobility group proteins homologous to the vertebrate HMG1 and HMG2 proteins preferentially recognize distorted DNA structures. DNA binding also induces a substantial bend. Using fluorescence resonance energy transfer (FRET), we have determined the changes in the end-to-end distance consequent on the binding of selected insect counterparts of HMG1 to two DNA fragments, one of 18 bp containing a single dA2 bulge and a second of 27 bp with two dA2 bulges. The observed changes are consistent with overall bend angles for the complex of the single HMG domain with one bulge and of two domains with two bulges of ~90-100°and ~180-200°, respectively. The former value contrasts with an inferred value of 150°reported by Heyduk et al. (1) for the bend induced by a single domain. We also observe that the induced bend angle is unaffected by the presence of the C-terminal acidic region. The DNA bend of ~95°observed in the HMG domain complexes is similar in magnitude to that induced by the TATA- binding protein (80°), each monomeric unit of the integration host factor (80°), and the LEF-1 HMG domain (107°). We suggest this value may represent a steric limitation on the extent of DNA bending induced by a single DNA- binding motif.