Nuclear particles sedimentation properties

SEDIMENTATION PROPERTIES OF PARTICLES. Different authors have given different S values for nuclear particles containing D-RNA. Determination of the sedimentation coefficients in a sucrose density gradient has been based on comparison with substances... 

The RNA of the mRNPs is sensitive to RNase and easily degraded. An increase in ionic strength leads to the dissociation of proteins from the particles. For example, in 0.55 M LiCl with 0.01 M MgCl2 the buoyant density of the particles increases and reaches 1.61 g/cm, which implies the loss of about three quarters of their total protein. A further increase in the salt concentration produces complete dissociation of the particles. After the removal of salt by dialysis the original particles can be reconstituted. The original and reconstituted particles have the same sedimentation properties and the same buoyant density (Perry and Kelley, 1968). Thus polysomal mRNP may be dissociated and reconstituted in a manner similar to that for nuclear complexes. 

Thus, for the purification of nuclear D-RNA from extracts, density gradient ultracentrifugation without prior pelleting is the best procedure at the moment. The material obtained from the peak with a sedimentation coefficient equal to 308 represents D-RNP particles of 90 to 95 percent purity and may be used for the study of the properties or function of the particles. 

Many properties of informosomes can be studied only after isolation of the particles in a pure state. As we can see, free informosomes of the embryonic cytoplasm resemble in some respects nuclear D-RNPs. They are relatively homogeneous with respect to buoyant density, at least in the case of large informosomes there is a good correlation between the sizes of the informosomes and the RNAs isolated from them. They form a number of discrete components in the sucrose density gradient. All the above-mentioned properties may be explained in terms of a polysomelike structure. However, for such a conclusion it is necessary to have data about the presence of minimal units in informosomes, which data are unavailable at the moment. On the other hand, there are some differences between the nuclear D-RNPs and informosomes with respect to their sedimentation coefficients. The question of the relation between the two types of particles is discussed in detail in the section on The Problem of Messenger RNA Transport (p. 96). 

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