Histone contributions tothestructure ofDNA inthenucleosome

ABSTRACT Wedescribe the application ofthe hydroxylradical footprinting technique to examinethe contribution ofthecorehistonetailsandofhistonesH3andH4tothestructureof DNAin the nucleosome. Wefirst establish that, as waspreviously determined for a nucleosome containing a uniquesequence of DNA, mixed-sequence nucleosomes contain twodistinct regions ofDNAstructure. Thecentral three turns ofDNAinthenucleosomehaveahelical periodicity of -10.7basepairs per turn, while flanking regions have a periodicity of-10.0base pairs perturn.Removalofthehistonetailsdoesnotchangethehydroxylradical cleavage pattern in either mixed-or unique-sequence nucleosome samples. Atetramer of his-tones H3and H4, (H3/H4)2, organizes the central 120 base pairs of DNA identically to that found in the nucleosome.Moreover, "tailless" octamers and the (H3/H4)2 tetramerrecognizethesamenucleosome positioning signalsastheintactoctamer.Many studieshaveinvestigatedtheroleofboth DNA andthehistoneproteins in thearchitecture ofthe