The structure of sub-nucleosomal particles. The octameric (H3/H4)4-125-base-pair-DNA complex

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Abstract
Chicken erythrocyte chromatin was depleted of histones H1, H5, H2A and H2B. The resulting (H3/H4)-containing chromatin was digested with micrococcal nuclease to yield monomer, dimer, trimer etc. units, irregularly spaced on the DNA, with even-number multimers being more prominent. Sucrose density gradient centrifugation separated monomers and dimers (7.7 S and 10.5 S). Sodium dodecyl sulphate gel electrophoresis and cross-linking indicated: (a) the monomer contains 50-base-pair (bp), 60-bp and 60-bp DNA and the dimer 125-bp DNA: (b) the monomer contains a tetramer and the dimer an octamer H3 and H4. Partial association of monomer units to dimers inhibits structural studies of monomers. The internal structure of the dimer, i.e. and (H3/H4)4-125-bp-DNA particle, was studied using circular dichroism, thermal denaturation and nuclease digestion. Both micrococcal nuclease and DNase I digestion indicate that, unlike core particles, accessible sites occur in the centre of the particle and it is concluded that the (H3/H4)4-125-bp-DNA particle is not a ''pseudo-core'' particle in which the ''extra'' H3 and H4 replace H2A and H2B. It is proposed that the octamer particle is formed by the sliding together of 2 ''monomer'' units, each containing the (H3/H4)2 tetramer and 70bp of DNA. Excision of this dimer unit with micrococcal nuclease results in the loss of 10 readily digestible base pairs at each end, leaving 125 bp.