Identification of a Rapidly Formed Nonnucleosomal Histone-DNA Intermediate that Is Converted into Chromatin by ACF

SummaryChromatin assembly involves the combined action of histone chaperones and ATP-dependent motor proteins. Here, we investigate the mechanism of nucleosome assembly with a purified chromatin assembly system containing the histone chaperone NAP1 and the ATP-dependent motor protein ACF. These studies revealed the rapid formation of a stable nonnucleosomal histone-DNA intermediate that is converted into canonical nucleosomes by ACF. The histone-DNA intermediate does not supercoil DNA like a canonical nucleosome, but has a nucleosome-like appearance by atomic force microscopy. This intermediate contains all four core histones, lacks NAP1, and is formed by the initial deposition of histones H3-H4. Conversion of the intermediate into histone H1-containing chromatin results in increased resistance to micrococcal nuclease digestion. These findings suggest that the histone-DNA intermediate corresponds to nascent nucleosome-like structures, such as those observed at DNA replication forks. Related complexes might be formed during other chromatin-directed processes such as transcription, DNA repair, and histone exchange.

Graphical AbstractFigure optionsDownload high-quality image (252 K)Download as PowerPoint slideHighlights
► Identification of a histone-DNA complex that is an intermediate in nucleosome assembly
► The ACF motor protein converts the histone-DNA intermediate into a canonical nucleosome
► This histone-DNA complex does not supercoil DNA, but resembles a nucleosome by AFM
► The intermediate may correspond to the initial form of “nascent” chromatin seen in vivo