Histone And DNA

The Detailed Arragement Of Histone And DNA In The Nucleosome

A Perspective On Current X-ray And Neutron Diffraction Analysis Of The Core Particle

            Complete solution of the structure of the nucleosome core particle which has a moleculer weight of about 200.000 daltons is a formidable task. The current X-ray diffraction analysis, with crystals diffracting to 0.5 nm in which there is one core particle per asymmetric unit²⁸, will permit the histone and DNA to be distinguished if isomophous heavy atom derivatives are required to solve the phase problem. The approach of choice to ensure that crystals of a derivatives are isomporphous with those of the unmodified material is to introduce heavy atoms by soaking the crystals in a solution of a suitable reagent (rather than crystallizing material modified in solution). Since the thiol group(s) of H3 appear to be inaccessible in the native state, the other obvious targets for attachement of heavy atoms are the ɛ-amino groups of lysine residues, if ways can be found restrict reaction to a limited number of sites, and to avoid disruption of possibly crucial electrostatic interactions of lysine ɛ-amino groups of lysine residues, if ways can be found to restrict reaction to a limited number of sites, and to advoid disruption of possibly crucial electrostatic interactions of lysine ɛ-amino groups and phosphates by introduction of bulky groups.

            Neutron diffraction studies with single crystals to resolution of 2.5 nm have been carried out¹¹ in parallel with the X-ray with the crystallography²⁸. The neutron analysis, with contrast matching of the very different scattering densities of alternately, protein and DNA by suspension of the crystals in the appropriate D₂O/H₂O mixtures, enables the DNA and proteinto be looked separately. The data corresponding to projections along the three crytal xes are in complete agreement with the result from electron microscopy and X-ray diffraction^{28, 29} and with model in which about 1.8 turns of a DNA superhelix of pitch 2.75 nm and radius 4.2 nm are wound around a protein core. This core has dimension about 5 x 5 x 6 nm and its projection along each of the three axes of the crystals is very similar to that of an independent three-dimensional recontruction of the histone octamer described below. If ways can be found to solve the phase problem (analogous to that encountered in X-ray crystallograph), future neutron analysis will also aim for a three-dimensional map of the nucleosome core particle.

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