Short-range DNA looping by the Xenopus HMG-box transcription factor, xUBF.

Xenopus UBF (xUBF) interacts with DNA by way of multiple HMG-box domains. When xUBF binds to the ribosomal promoter, the carboxyl-terminal acidic tail and amino-terminal HMG-box interact. Binding also leads to negative DNA supercoiling and the formation of a disk-like structure, the enhancesome. Within the enhancesome, an xUBF dimer makes a low-density protein core around which DNA is looped into a single 180-base pair turn, probably by in-phase bending. The enhancesome structure suggests a mechanism for xUBF-dependent recruitment of the TATA box-binding protein complex without direct interaction between the two factors.