The DNA supercoiling architecture induced by the transcription factor xUBF requires three of its five HMG-boxes

The formation of a near complete loop of DNA is a striking property of the architectural HMG-box factor xUBF. Here we show that DNA looping only requires a dimer of Nbox13, a C-terminal truncation mutant of xUBF containing just HMG-boxes 1-3. This segment of xUBF corresponds to that minimally required for activation of polymerase I transcription and is sufficient to generate the major characteristics of the footprint given by intact xUBF. Stepwise reduction in the number of HMG-boxes to less than three significantly diminishes DNA bending and provides an estimate of bend angle for each HMG-box. Together the data indicate that a 350 +/- 16 degrees loop in 142 +/- 30 bp of DNA can be induced by binding of the six HMG-boxes in an Nbox13 dimer and that DNA looping is probably achieved by six in-phase bends. The positioning of each HMG-box on the DNA does not predominantly involve DNA sequence recognition and is thus an intrinsic property of xUBF.