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R M Stephens

Publications and source records attributed to R M Stephens.

45 records · Page 3Linked to original sources
PubMed1977

A nucleosome-like structure containing DNA and the arginine-rich histones H3 and H4.

The low-angle X-ray diffraction pattern from fibres of reconstituted H3/H4/DNA complexes is very similar to that of chromatin and has well defined maxima at 10.6, 5.4, 3.4 and 2.6 nm. Staphyloccal nuclease digestion of reconstituted H3/H4/DNA yields DNA fragments of length 49, 69, 100, 128, 193 and 255 b.p. as principal components. Comparison of the relative amounts of DNA fragments shows that the larger components (100 and 128 b.p.) increase with respect to the smaller (49 and 69 b.p.) as the histone to DNA ratio increases. A structural unit containing intergral of 65 b.p. of DNA and tetrameric (H3/H4)2 is proposed such that longer DNA fragments result from multiples of this unit. The principal nucleo-protein particle resulting from nuclease digestion contains 128/139 b.p. of DNA and has electrophoretic mobility very close to that of 'core' nucleosome. It probably represents a dimer of the basic structural unit.

Animals↗
PubMed1976

Physical studies of chromatin. The recombination of histones with DNA.

Experiments have been carried out to define clearly which histone combinations can induce a higher order structure when combined with DNA. The criterion for a higher order structure being the series of low-angle X-ray diffraction maxima nominally at 5.5 nm, 3.7 nm, 2.7 nm and 2.2 nm. Such a pattern, with resolution similar to that of H1-depleted chromatin, is readily attainable by recombining histones H2A + H2B + H3 + H4 with DNA using a salt-gradient dialysis method. However, the use of urea in the recombination procedure is shown to be detrimental to the production of a higher order structure. Low-angle ring patterns are not obtained by recomgining DNA with single pure histones or any combination of histone pairs exept H3 + H4. The diffraction maxima from the latter are, however, weaker than those from chromatin and there are pronounced semi-equatorial arcs. The presence of a third histone, either H2A or H2B in the H3 + H4 recombination mixture tends to distort the recognised low-angle pattern. It is concluded that the histone pair H3 + H4 is essential for the formation of a regular higher order structure in chromatin, although for a complete structural development the presence of H2A + H2B is also required.

Binding Sites↗
PubMed1970

The dissociation of chicken erythrocyte deoxyribonuleoprotein and some properties of its partial nucleoproteins.

Histones were completely dissociated from their native complex with DNA in 2.0m-sodium chloride. Histone fractions IIb, V and I were dissociated in 1.2m-sodium chloride, fractions V and I in 0.7m-sodium chloride and fraction I in 0.45m-sodium chloride. Repeated extraction of partial dRNP (deoxyribonucleoprotein) preparations with sodium chloride of the same concentration as that from which they were prepared resulted in release of histones that previously had remained associated with the DNA of the complex. Gradual removal of histones from dRNP was paralleled by an improvement in solubility, a decrease in wavelength of the u.v.-absorption minimum, and a fall in sedimentation coefficient of the remaining partial dRNP. X-ray diffraction patterns of partial dRNP preparations showed that removal of histone fractions I and V from dRNP did not destroy the super-coil structure of the dRNP, but further removal of histones did. Infrared spectra of partial dRNP preparations showed that in native dRNP histone fraction I was present in the form of extended, isolated polypeptide chains, and that the other histone fractions probably contain a helical component that lies roughly parallel to the polynucleotide chains in the double helix and an extended polypeptide component that is more nearly parallel to the DNA helix axis. An analysis of the sedimentation of partial dRNP preparations on sucrose gradients showed that native dRNP consists of DNA molecules each complexed with histone fractions of all types.

Amino Acids↗