Polysomelike particles

The constituents of polysomelike particles (Samarina et al, 1968a, b Molnar et al,... 

To answer the first question, the protein composition of the heavy complexes obtained from different zones of the sucrose density gradient (60 to SOS and 100 to 120S) was compared with that of the SOS particles by gel electrophoresis. Only the bands found in extracts of the SOS particles were observed in polysomelike particles no ribosomal proteins are found to contaminate the heavy particles. This means that poly particles consist of SOS particles exclusively and do not contain ribosomes, ribosomal subunits, or other cellular components. 

To check whether particles heavier than 308 have a polysomelike structure, that is, consist of several 308 particles joined by a D-RNA strand, the effect of RNase action on the nuclear extracts isolated with the RNase inhibitor present was studied. With an optimal dose of RNase it is possible to convert the radioactive material of the heavy peaks almost quantitatively into the 308 particles (8amarina et al., 1967e 1968a, b) (Fig. 2). 

These data prove the polysomelike structure of the nuclear particles containing D-RNA. The 308 particle seems to be a monomer of this complex structure. 

The question arises whether only SOS particles form these polysomelike complexes, or whether other structural elements also are involved. In addition, how tightly are the protein particles packed along the D-RNA strand ... 

Another approach is to measure the buoyant density of the particles using CsCl density gradient ultracentrifugation. If the strands of RNA between the SOS particles in the polysomelike complexes are short (as may be expected from electron micrographic evidence) the ratio of RNA to protein and consequently the buoyant density of the heavy particles and of SOS particles should be the same. If the RNA strands between particles are long, this ratio and the density should be higher. The experiments performed with... 

These data provide additional proof of the polysomelike structure of the D-RNA-containing complexes. On the other hand, these data as well as electron microscopic observations allow us to estimate some of the characteristics of poly particles. 

The internal structure of these large particles has not yet been resolved by electron microscopy, but it is clear that they are not homogeneous. It is quite possible that they represent conglomerates of 200 A particles similar to those observed in the experiments of Samarina et al. (1968b) with isolated particles. In the latter case the polysomelike complexes are very often not extended but clumped, and it is impossible to recognize any internal structural organization. For this reason one cannot exclude the possibility that the Beermann particles are polysomelike complexes between D-RNA and informofers. It was shown further that the particles are formed near the chromatin masses from rod-shaped structures (which may be chains of informofers). Different steps in the conversion of rods of 400 A particles may be found. 

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). 

The comparison of nuclear and cytoplasmic D-RNPs. In the above sections we described the properties of different types of cellular D-RNP. If nuclear D-RNA-containing particles and free informosomes are compared one can find a number of similarities. Both kinds of particles give several discrete peaks in a sucrose density gradient, which, at least in the case of nuclear D-RNPs reflects the polysomelike structure of the complexes. 

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