Nucleosomal conformations induced by the small HMG proteins or by histone hyperacetylation are distinct.

Nucleosomal particles with a reduced electrophoretic mobility can arise from the presence of HMG proteins 14 and 17 or from hyperacetylating the histone core. Both forms have been prepared from Namalva (Burkitt lymphoma) cells. After deacetylation, sequences of the inducible but nontranscribed interferon-beta genes are still part of the low mobility class of particles suggesting that they carry a member of the small HMG proteins. A comparison of HMG-bonded and hyperacetylated particles on density gradient gels shows that in the first case slow mobilities arise from a reduced effective charge and in the second from an increased friction, i.e., a relaxed nucleosome structure. The interaction of HMG 14 with compact and relaxed nucleosomes has been compared to appreciate the role of histone acetylation. It is shown that hyperacetylation reduces the affinity and cooperativity of binding HMG and may be a prerequisite for an efficient transcription.