Conformation of Chromatin Oligomers

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Abstract
Quaiselastic laser light scattering measurements were made on chromatin oligomers to obtain information on the transition in their electrooptical properties, previously observed for the hexameric structures. Translational diffusion coefficients were determined for mononucleosomes to octanucleosomes containing histone H1 over a range of ionic strength. At high ionic strength, oligomers show a linear dependence of the logarithm of diffusion coefficient upon the logarithm of number of nucleosomes. At low ionic strength a change occurs between hexamer and heptamer. The results agree well with the recent sedimentation data of Osipova et al. and of Butler and Thomas showing a change in stability with hexamer. Various models for the arrangements of nucleosomes in the superstructure of chromatin are discussed. All calculations clearly indicated a conformational change with the hexanucleosome and at low ionic strength, the chromatin apparently adopts a loosely helical structure of 28 nm diameter and 22nm pitch. These results are also consistent with a discontinuity every 6th nucleosome, corresponding to a turn of the helix. This discontinuity may explain the recent electric dichroism data of Lee, et al. The hexanucleosome structure which was previously suggested, with the faces of nucleosomes arranged radially to the helical axis, was recently confirmed by Mc Ghee et al. With an increase of ionic strength, the helix becomes more regular and compact with a slightly reduced outer diameter and a decreased pitch, the dimensions resembling those proposed for solenoid models.