Prenucleosomes and Active Chromatin.

Chromatin consists of nucleosomes as well as nonnucleosomal histone-containing particles. Here we describe the prenucleosome, which is a stable conformational isomer of the nucleosome that associates with ∼80 bp DNA. Prenucleosomes are formed rapidly upon the deposition of histones onto DNA and can be converted into canonical nucleosomes by an ATP-driven chromatin assembly factor such as ACF. Different lines of evidence reveal that there are prenucleosome-sized DNA-containing particles with histones in the upstream region of active promoters. Moreover, p300 acetylates histone H3K56 in prenucleosomes but not in nucleosomes, and H3K56 acetylation is found at active promoters and enhancers. These findings therefore suggest that there may be prenucleosomes or prenucleosome-like particles in the upstream region of active promoters. More generally, we postulate that prenucleosomes or prenucleosome-like particles are present at dynamic chromatin, whereas canonical nucleosomes are at static chromatin.