In vitro assembly of a positioned nucleosome near the hypersensitive region in simian virus 40 chromatin.

Previous studies have identified a nucleosome near a potential late boundary for the nuclease-hypersensitive region in simian virus 40 chromatin. We have performed in vitro reconstitution analysis to determine whether the underlying DNA sequence encodes for the assembly of this nucleosome and applied hydroxyl radical and DNase I footprinting techniques to examine the structure of the reconstituted nucleosome. Both methods revealed the formation of a precisely positioned nucleosome in vitro, on a fragment spanning the strong in vivo nucleosome location site determined previously in the viral chromatin. The center of the positioned nucleosome maps between nucleotide 384 and 387 on simian virus 40 DNA. The corresponding nucleosome core includes the major-late transcription site (12 base-pairs within the core), the MspI site, and a segment shown previously to adopt a bent structure in the absence of proteins. The hydroxyl radical produces a strikingly well-defined cleavage pattern over the bent DNA incorporated in nucleosomes. The dominant periodicity of DNA in this nucleosome is 10.26 base-pairs per turn. The distribution of the .OH cut sites in the positioned nucleosome provides strong support for models in which the minor grooves of the A/T-rich tracts are oriented toward the histone core while the minor grooves of the G/C-rich sequences are facing outward.