Affinity of HMG17 for a mononucleosome is not influenced by the presence of ubiquitin-H2A semihistone but strongly depends on DNA fragment size.

We have used a two-dimensional (deoxyribonucleoprotein leads to DNA) electrophoretic binding assay to study the interaction of the purified high mobility group protein HMG17 with isolated HeLa mononucleosomes as a function of their DNA fragment size and the presence of ubiquitin-H2A semihistone. No significant differences between affinities of HMG17 for ubiquitinated and non-ubiquitinated core mononucleosomes were observed. In striking contrast, the apparent affinity of HMG17 for a mononucleosome increases more than 100-fold upon an increase of the length of the mononucleosomal DNA fragment by as few as 3 to 5 bp over the core DNA length (integral of 146 bp). We suggest that the magnitude of this effect is sufficient to explain the preferential binding of HMG17 in vitro to mononucleosomes derived from actively transcribed genes.