DNA looping in the RNA polymerase I enhancesome is the result of non-cooperative in-phase bending by two UBF molecules

The so-called upstream binding factor (UBF) is required for the initial ste p in formation of an RNA polymerase I initiation complex. This function of UBF correlates with its ability to induce the ribosomal enhancesome, a stru cture which resembles in its mass and DNA content the nucleosome of chromat in. DNA looping in the enhancesome is probably the result of six in-phase b ends induced by the HMG boxes of a UBF dimer. Here we show that insertion/d eletion mutations in the basic peptide linker lying between the N-terminal dimerisation domain and the first HMG box of Xenopus UBF prevent the DNA lo oping characteristic of the enhancesome. Using these mutants we demonstrate that (i) the enhancesome structure does not depend on tethering of the ent ering and exiting DNA duplexes, (ii) UBF monomers induce hemi-enhancesomes, bending the DNA by 175 +/- 24 degrees and (iii) two hemi-enhancesomes are precisely phased by UBF dimerisation. We use this and previous data to refi ne the existing enhancesome model and show that HMG boxes 1 and 2 of UBF li e head-to-head along the DNA.