High-mobility-group 1 protein mediates DNA bending as determined by ring closures

High-mobility-group 1 protein (HMG1) is an abundant eukaryotic DNA-binding protein, the cellular role of which remains ill-defined. To test the ability of HMG1 itself to mediate curvature in double-stranded DNA, we examined its effect on the phage T4 DNA ligase-dependent cyclization of short DNA fragments. HMG1 caused circle formation for fragments > or = 87 bp. Fragments of 123, 100, 92, and 87 bp did not cyclize in the absence of protein but formed covalently closed circular monomers efficiently in the presence of HMG1, indicating that the protein is capable of introducing bends into the duplex. The bending activity was maintained by a 79-amino acid polypeptide corresponding to a single HMG-box domain of HMG1. The binding affinity for the DNA minicircle was greater than for the corresponding linear fragment. These findings indicate that the role of HMG1 could involve both structure-specific recognition of prebent DNA and distortion of the DNA helix by bending and that the HMG-box domain may actually be responsible for this activity.