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The crystal structure of the nucleosome core particle has been studied by neutron diffraction to a resolution of 16 A. By using H2O/D2O solvent contrast variation, the structures of the DNA and histone core were analysed separately. The DNA, as seen at this resolution, forms a super-helix of pitch 25.8 A, radius 42.1 A and 1.8 turns in length. The histone core itself is approximately helical and follows the DNA along the inside of the super-helix, giving the nucleosome core particle an overall 2-fold axis of symmetry. Four regions can be distinguished in the protein density, which we interpret as dimers of histones within the octameric core. The dimers have been assigned on the basis of other evidence as being of two kinds, (H2A-H2B) and (H3-H4). Because solvent contrast variation can distinguish between hydrophobic and hydrophilic regions in the protein density, our results suggest that the interface between the monomers of each dimer is probably quite hydrophobic in character, while the interaction between dimers is weaker and/or more hydrophilic. The protein is in contact with most of the DNA and there are some regions where it may penetrate between the turns of the super-helix. In particular, the tetramer (H4-H3)-(H3-H4) is in close contact with the central part of the DNA, but significant contacts are seen also between the histones H3 and the extremities of the super-helix, thus explaining the stability of a nucleosome-like particle depleted of H2A and H2B. Significant departures from the molecular 2-fold axis of symmetry occur in the relative arrangements of the two (H2A-H2B) dimers.

The nucleosome subunits of chromatin are assembled from histones and DNA by an acidic protein which binds histones. The nucleosome assembly protein has been identified and purified from eggs of Xenopus laevis.

Poly(dA).poly(dT) has unusual properties in that it cannot associate into nucleosomes and short, phased runs of it cause DNA bending. The crystal structure of a B-type DNA dodecamer containing a homopolymeric run of six A.T base pairs shows that this region possesses special structural features, including a system of bifurcated hydrogen bonds, which explains some of the properties of this simple homopolymer.

Segments of cloned cDNA to tobacco mosaic virus RNA, 150--300-bases long, have been hybridised and cross-linked to the RNA, which has then been used for reassembly experiments. This enables the elongation reaction, which does not encapsidate the double-stranded region generated, to be stopped at specific regions along the RNA and the resulting particles to be characterised, by measuring the lengths of the rods in the electron microscope. With hybridisation to the 3'-tail the entire RNA contiguous to the nucleation region is encapsidated, from the 5'-terminus up to the modified region. When the double-stranded region is on the 5'-side of the nucleation region, the mean length of the particles corresponds to a situation in which the double-stranded region is unable to enter the central hole of the growing rod, but the 3'-tail of the RNA is completely encapsidated. The longest particles hybridised on the 5'-tail (i.e. in a class longer than the mean length) show an effect complementary to those with a 3'-block, and have lengths which correspond to encapsidation from the modified region to the 3'-terminus, despite the continued presence of the 5'-tail up the rod. In all cases where there is a remaining 5'-tail the lengths observed can only be explained if elongation has occurred substantially, or probably completely, along the 3'-tail. Hence elongation must have occurred simultaneously along both the 5' and 3'-tails of the tobacco mosaic virus RNA after initiation on the internal nucleation region.

A comparison is made between the electron density maps of the monoclinic and orthorhombic crystal forms of yeast tRNA(Phe) which have been obtained respectively by ourselves and by another group. It is concluded that the molecular structures are essentially the same in both crystals, although the models derived from the maps are not the same. The relation between the two molecular packings is discussed, and it is suggested that the intermolecular contact in the orthorhombic form which is not present in the monoclinic form, may arise through base pairing of the anticodons of neighboring molecules.

Binding of the oligoribonucleotides AAG, AAGAAG and AAGAAGUUG to the disk aggregate of tobacco mosaic virus coat protein has been studied in solution under conditions favourable for virus assembly. The two longer oligomers bind strongly with Kd around 1 microM, approach complete saturation of binding sites and cause the formation of long, nicked helical rods resembling the virus. It is suggested that the binding of these oligomers, with sequences chosen from the assembly origin of the viral RNA, simulates the tobacco mosaic virus assembly process. No binding could be detected for AAG, indicating that chain length is a crucial determinant in the interaction. The binding of AAGAAG to coat protein crystals is very much weaker than that observed in solution, and the crystals crack at high oligomer concentrations. The corresponding oligodeoxyribonucleotide, d(AAGAAG), shows no binding to the protein in solution; the interaction is extremely specific for RNA.

When TMV reassembles, the uncoated RNA is folded back along the growing rod, probably down the central hole. This surprising configuration is essential for rapid elongation--presumably supplying RNA to its site of incorporation while keeping the bulk of the free RNA out of the way.

Subgenomic RNAs (sgRNAs) were isolated from defective interfering virus produced by high multiplicity passage of the human influenza strain A/PR/8/34. Cloning and sequencing of 35 unique sgRNAs revealed that many were about 400 nucleotides long, containing about 200 nucleotides from each of the 5' and 3' ends of a full-length segment. Most of the sgRNAs were derived from segment 1, but there were examples from six other segments, including those encoding the haemagglutinin and neuraminidase. Our analysis of the sequence rearrangements found in sgRNAs indicates that they may be generated from the standard viral segments by a jumping viral polymerase that makes transitions between adjacent regions of the RNA template in the ribonucleoprotein tertiary structure.

Cystals have been obtained on nucleosome cores and analysed by X-ray diffraction and electron microscopy. The core is a flat particle of dimensions about 110 X 110 X 57 A, somewhat wedge shaped, and strongly divided into two 'layers', consistent with the DNA being wound into about 1 3/4 turns of a flat superhelix of a pitch about 28 A. The organisation of the DNA can be correlated with the results to enzyme digestion studies. A change in the screw of the DNA double helix on nucleosome formation can be deduced.

The crystal structure of the nucleosome core particle has been solved to 7 A resolution. The right-handed B-DNA superhelix on the outside contains several sharp bends and makes numerous interactions with the histone octamer within. The central turn of superhelix and H3 . H4 tetramer have dyad symmetry, but the H2A . H2B dimers show departures due to interparticle associations.

Deoxyhemoglobin from patients homozygous for sickle-cell anemia (deoxyhb S) aggregates into long straight fibers. These may extend through most of the length of the sickled cell, forming either square or hexagonally packed bundles with lattice constants of 170-180 A. Each fiber is a tube made up of six thin filaments, which are wound around the tubular surface with a helical pitch of about 3000 A. Each filament is a string of single hemoglobin molecules linked end to end at intervals of 62 A in dry and 64 A in wet fibers. Molecules in neighboring filaments are in longitudinal register so that they form flat hexagonal rings; these rings are stacked so that successive ones are rotated about the fiber axis by 7.3 degrees . The whole structure repeats after about eight rings. In this structure each molecule makes contact with four neighbors. The likely orientation of the molecules and points of contact between them are discussed. Similar filaments are also observed in normal deoxygenated erythrocytes, but in much lower concentration and aggregated into fibers of irregular diameter. No filaments appear in oxygenated sickle, or normal, adult cells, nor in oxygenated or deoxygenated fetal cells.