PubMed · 5445317
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D R Harvey. 1970. Reorienting gradient technique for continuous-flow centrifugation.. https://doi.org/10.1016/0003-2697(70)90318-0
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RNA b is the most abundant member of a family of autonomously replicating single- and double-stranded RNA plasmids found in maize mitochondria. The extent to which this molecule is associated with proteins was investigated by rate zonal and CsCl equilibrium density gradient centrifugation of clarified lysates of S cytoplasm maize mitochondria. A soluble complex of RNA b, responsible for synthesis of the more abundant (+) RNA b strand in mitochondrial lysates, was identified. The complex had a buoyant density of 1.49 g/cm3, indicating a substantial non-nucleic acids content. The sedimentation coefficient of the complex, however, was only slightly larger than that of deproteinized RNA b. Synthesis of RNA b as well as the larger RNA plasmid, RNA a, was resistant to heparin, suggesting that, for both RNAs, preformed complexes between an RNA template and an RNA-dependent RNA polymerase capable of elongating in vivo preinitiated RNA plasmid strands, were present in the lysate. Only a small fraction of RNA b molecules were bound in the complex; the bulk of RNA b sedimented at the same rate as the deproteinized RNA. Thus, after replication, maize mitochondrial plasmids are not associated with nucleoprotein capsids although their synthesis takes place through ribonucleoprotein replication complexes.
Low-density lipoproteins (LDL) in plasma are constructed from a single molecule of apolipoprotein B-100 (M(r) 512000) in association with lipid (approximate M(r) 2-3 x 10(6)). The gross structure was studied using an updated pulsed-neutron camera LOQ with an area detector to establish the basis for the interpretation of structural changes seen during dynamic studies of LDL oxidation. Neutron-scattering data for LDL in 100% 2H2O buffers emphasize their external appearance. Guinier analysis on a continuous-flux neutron camera D17 revealed pronounced concentration-dependences in the radius of gyration, RG, and the intensity of forward scattering, I(0) (equivalent to the M(r) of LDL) between 0.5 and 11 mg of LDL protein/ml. LDL preparations from different donors gave different RG values. When extrapolated to zero concentration, RG values ranged between 8.3 and 10.6 nm and were linearly correlated with M(r), which is consistent with a spherical structure. The distance-distribution function P(r) in real space showed a single maximum at 9.1-10.9 nm, which is just under half the observed maximum dimension of 23.1 +/- 1.2 nm expected for a spherical structure. The neutron radial-density function p(r) exhibited a plateau of high and featureless density at the centre of LDL. LDL can be modelled by a polydisperse assembly of spheres with two internal densities and a mean radius close to 10.0 nm in a normal distribution of radii with a standard deviation of 2.0 nm. The data are consistent with recent electron-microscopy and ultracentrifugation data.(ABSTRACT TRUNCATED AT 250 WORDS)
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