DNA-BINDING PROPERTIES OF THE TANDEM HMG BOXES OF HIGH-MOBILITY-GROUPPROTEIN-1 (HMG1)

High-mobility-group protein 1 (HMG1) is a conserved chromosomal protei n with two homologous DNA-binding HMG-box domains, A and B, linked by a short basic region to an acidic carboxy-terminal tail. NMR spectrosc opy on the free didomain (AB) shows that the two HMG boxes do not inte ract. The didomain has a higher affinity for all DNA substrates tested than single HMG-box domains and has a significantly higher ability to distort DNA by bending and supercoiling. The interaction of the didom ain with DNA is stabilized by the presence of the basic region (approx imate to 20 residues, 9 of which are Lys) that links the second HMG bo x to the acidic tail in intact HMG1; this may be, at least in part, wh y this region also enhances supercoiling of relaxed circular DNA by th e didomain and circularization of short DNA fragments (in the presence of ligase). Competition assays suggest significantly different struct ure-specific preferences of single and tandem HMG boxes for four-way j unction and supercoiled plasmid DNA. Binding to supercoiled DNA appear s to be promoted by protein oligomerization, which is pronounced for t he didomains. Electron microscopy suggests that the oligomers are glob ular aggregates, associated with DNA looping. One box versus two (or s everal) is likely to be an important determinant of the properties of (non-sequence specific) HMG-box proteins.