Structure of F-pili: reassessment of the symmetry.

Reassessment of the X-ray fibre diffraction patterns of F-pili using a more accurate subunit molecular weight suggests that subunits in F-pili are related by a fivefold rotation axis around the pilus axis. The identity of this fivefold symmetry with the fivefold rotation axis that relates the subunits in fd bacteriophage supports a simple model for tip-to-tip adsorption of bacteriophage to pili.

Packing of collagen molecules modified with 2-propanol.

X-ray diffraction data of collagen molecules modified with 2-propanol favour a quasi-hexagonal lateral packing over a quasi-tetragonal one.

Stress-induced molecular rearrangement in tendon collagen.

Tension-induced molecular rearrangements in wet native fibres of rat-tail tendons and human finger flexor tendons are registered with the help of time-resolved diffraction spectra using synchrotron radiation. The tension-induced increase of the 67 nm D period is combined with changes in the intensities of some orders of the meridional small angle reflection. Both effects are reversible when unloading the fibre, but are preserved when the load is held constant until the fibre tears. The increase in the D period is partly due to a sliding of the triple helices relative to each other and partly due to a stretching of the triple helices themselves. The sliding of the triple helices results in an alteration of the D stagger, leading to a change in the length of the gap and overlap regions, and to a stretching of the cross-linked telopeptides. This interpretation is supported by comparison with the relative intensities derived from a model with varying length of gap and overlap regions, as well as by comparison with model calculations that include the telopeptides.

Twisted fibrils are a structural principle in the assembly of interstitial collagens, chordae tendineae included.

X-ray diffraction analysis of connective tissue samples, which contain type I and type III collagen shows that twisted collagen fibrils are a general principle of assembly. The occurrence of twisted fibrils in native wet Chordae tendineae, skin and Aorta is combined with a shorter axial periodicity of about 65 nm. This shorter D period is shown to be directly related to the tilt of the molecules, which have to be curved to build-up twisted fibrils.

The macromolecular structure of collagen in tendon fibres of dermatosparactic animals.

Small-angle x-ray diffraction spectra of dermatosparactic tendon collagen show a decreased intensity of the first order reflection. We interprete this finding to be due to the N-terminal propeptide which fills the intermolecular gap region partially.

Small-angle X-ray and light scattering studies on the influence of Mg2+ ions on the structure of the RNA from bacteriophage MS2.

The influence of Mg2+ ions on the secondary and tertiary structure of the RNA from bacteriophage MS2 was investigated by small-angle X-ray scattering and light scattering and by sedimentation experiments. The analysis of the outer part of the X-ray scattering curve obtained at low temperature in the absence of Mg2+ yielded a cross-section radius of gyration of 0.88 nm and a mass per unit length of 1720 g mol-1 nm-1. Very similar values for these parameters, which refer to the secondary structure of the RNA molecule, were also derived from the X-ray scattering curves obtained in the presence of different amounts of Mg2+ (0.07 to 1 ions per nucleotide). On the contrary, the inner part of the X-ray scattering curves turned out to be highly dependent on the Mg2+ concentration: the cross-section radius of gyration and the mass per unit length, which were determined from the scattering curves at small angles as parameters related to the tertiary structure of the RNA, amounted to 3.11 nm and 4000 g mol-1 nm-1, respectively, in the absence of Mg2+ and increased significantly upon raising the concentration of Mg2+. The increase of these structural parameters was found to be accompanied by a decrease of the overall radius of gyration (as revealed indirectly by X-ray scattering and directly by light scattering measurements) and by an increase of the sedimentation coefficient. The results from the investigations of the RNA at low temperature clearly establish the existence of double-stranded structures down to very low Mg2+ concentrations as well as the occurrence of Mg2+ induced changes of the tertiary structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Maximum-entropy calculation of the electron density at 4 A resolution of Pf1 filamentous bacteriophage.

A 4 A electron-density map of Pf1 filamentous bacterial virus has been calculated from x-ray fiber diffraction data by using the maximum-entropy method. This method produces a map that is free of features due to noise in the data and enables incomplete isomorphous-derivative phase information to be supplemented by information about the nature of the solution. The map shows gently curved (banana-shaped) rods of density about 70 A long, oriented roughly parallel to the virion axis but slewing by about 1/6th turn while running from a radius of 28 A to one of 13 A. Within these rods, there is a helical periodicity with a pitch of 5 to 6 A. We interpret these rods to be the helical subunits of the virion. The position of strongly diffracted intensity on the x-ray fiber pattern shows that the basic helix of the virion is right handed and that neighboring nearly parallel protein helices cross one another in an unusual negative sense.

Small-angle x-ray studies on the structure of 16-S ribosomal RNA and of a complex of ribosomal protein S4 and 16-S ribosomal RNA from Escherichia coli.

16-S ribosomal RNA and a complex of ribosomal protein S4 and 16-S rRNA were studied in solution by small-angle X-ray scattering. Concentration series of the 16-S rRNA and the S4 - 16-S-rRNA complex were measured in 37.5 mM Tris-HCl buffer pH 7.4 at 5 degrees C. The following data were determined. The radii of gyration for the 16-S rRNA and S4 - 16-S-rRNA complex were R = 17.6 +/- 0.6 nm, respectively. The two respective values of the radii of gyration of the cross-section were Rq,1 = 8.42 +/- 0.1 nm and 8.33 +/- 0.3 nm, and Rq,2 = 0.988 +/- 0.03 nm and 0.996 +/- 0.03 nm. The largest diameters of the 16-S RNA and S4 - 16-S-RNA complex were L = 61.8 +/- 1 nm and 60.0 +/- 1 nm, respectively. Volumes of V = 1570 +/- 60 nm3 were found for both particles. In the Tris buffer used, no significant differences were found between the scattering curves of 16-S rRNA and the complex is a flat elliptical cylinder with the following dimensions: large axis 61.7 nm, small axis 35.4 nm and height 2 nm. The theoretical scattering curve fits the experimental one as long as the shape of the measured curve is due only to the overall shape of the particle. A model equivalent over the whole measured angular range is one built up from a large number of spheres that simulate the known substructure of the RNA. The outer dimensions of this model correspond to those of the flat elliptical cylinder.

[Small-angle X-ray-scattering investigation and structural-model study of the fatty-acid synthetase from pig liver (author's transl)]. / Röntgenkleinwinkel-Untersuchungen und Strukturmodell-Betrachtungen an der Fettsäuresynthetase aus Schweineleber.

The structure of the fatty acid synthetase from pig liver was studied on models based upon structural and functional properties selected from pertinent results available from numerous investigations carried out with fatty acid synthetases from this and other sources. When comparing small-angle X-ray-scattering curves calculated with these models and curves obtained from small-angle X-ray-scattering experiments carried out with the pig-liver enzyme, we tried to select a model which would lead to an acceptable correlation between the calculated and the experimental curves and at the same time fulfil the known structural and functional requirements. The comparison of the curves was started with a model of low complexity. The observed discrepancy, together with arguments from the structural and the functional properties, helped decide which is the next most reasonable model to be considered. This procedure was repeated for five models of increasing complexity. In the model which led to the best fit the multienzyme complex is composed of two halves in an assymetric conformation including hollow spaces. This highly anisotropic model would imply that the two halves change their conformation each time a synthetic cycle is completed and that the growing fatty acid is handed over from one half to the other.

Ehlers-Danlos syndrome type IV (EDS IV) as model of a defective biopolymer composite material.

The connective tissue of a lethal EDS IV case was investigated for the reasons of the manifested disturbances of the arterial wall. This functional disorder was attributed to the mechanical decoupling of elastin and collagen, with the premise of a composite material consisting of cellular, fibrillar, lamellar and other matrix components. A conceivable relation between the manifested deficiency of type III collagen and a disturbed anchoring of elastin is shown. These findings are supported by biochemical, morphological, x-ray and mechanical data.

New aspects of the etiology of tendon rupture. An analysis of time-resolved dynamic-mechanical measurements using synchrotron radiation.

Native collagen fibers were exposed to different dynamic loads to simulate damage to tendons and ligaments relevant clinically and for sports medicine. The results suggest that the rupture of a tendon is caused at the submicroscopic fibrillar level. Not only slow or very fast elongation, but also very fast unloading of stretched fibers seems to be responsible for disseminated damage, which reduces the stability of a fiber. This damage is induced by intrafibrillar sliding processes, which occur only a few seconds before macroscopic slippage takes place. The significance of these events for the beginning and progress of repair in vivo is discussed. The conclusions are supported by simultaneous mechanical and radiological measurements, as well as by light- and electron-microscopic results.