Particles of Nuclear Origin Carrying Rapidly Labelled RNA

Abstract

Nuclei of KB cells have been isolated and incubated at 37" in pH 8.0 saline. The experia) Isolated nuclei release well defined nucleoprotein particles, which contain RNAs not deb) These particles carry rapidly labelled RNA. c) In cytoplasm they cosediment with the long known smaller ribosomal particles. d) Chase experiments suggest that nuclear particles are precursors of cytoplasmic particles. ments described in this paper provide evidence that: tectable in untreated nuclei.In higher organisms the bulk of cellular RNA, e. g. 25 S and 18 S ribosomal RNAs, messenger-and transfer RNAs, are synthesized within the nucleus on the chromosomes [l]. The center of activity of these RNAs, namely the polysomes, are on the other hand exclusively located in the cytoplasm. Ribosomal and messenger RNAs of the same sizes as those obtained from polysomes are not found in the nucleus. I n contrast, nuclei bf mammalian cells contain types of RNA having a relatively high sedimentation rate (> 30 S), which is supposed to reflect a high molecular weight and a great chain length. Such molecules, however, are not found in the cytoplasm [ 2 ] . Previous research has accumulated evidence that RNA has to pass the nuclear membrane in order to get to the cytoplasm. It is then apparently on this passage that such RNA is cut to the required sizes [3,4].In the cytoplasm newly synthesized rapidly labelled RNAs, do not appear as free molecules, but are bound to small particles, e . g. either pre-ribosomal particles or informosomes [3,5-81. The synthesis and/or the passage of the M e r e n t RNA species differs strongly. The smaller 45 S subunits of the ribosomes are labelled faster with radioactive precursors than the larger 60 S subunits; it is obvious that the two enter the cytoplasm independently, and that the 45 S particle is transferred a t a higher rate.This could imply that it is synthesized a t higher rates also.From our own work [6] and from experiments of Girard and collaborators [9], it is further evident that messenger-RNA is the very first to leave the nucleus.Unusual Abbreviations. niRNA, messenger RNA; tRNA. transfer RNA.Note. The sedimentation coefficients of the particles and RNA species discussed here are approximate and were not dotermincd. The values are adopted from the publications cited. They are used to name the hitherto nameless particles and RNA species.

Characterization of Pre‐messenger‐RNA‐Containing Nuclear Ribonucleoprotein Particles from Avian Erythroblasts

“…In general about 35 % of the radioactivity in purified nuclei is released which is in good agreement with previously reported values [31,32]. The progressive aggregation of nuclei provoked by these conditions produces some variation in the final yield of nuclear ribonucleoprotein.…”

The protein moieties of animal messenger ribonucleoproteins

“…This led Olsnes and Pihl to suggest that the function of the informofer protein is to protect the mRNA from non-specific binding to ribosomes. The 2 nuclear mRNP co-sediments on sucrose gradients with the small ribosomal subunit after incubation with KB cell cytoplasm [24] .…”

Messenger RNA in HeLa Cells:

“…It is evident that protein molecules would be ideal for such a mechanism in view of their capability to interact with macromolecules like DNA, RNA and the constituents of cell membranes, as well as with small molecular weight effectors inducing allosteric transitions. Samarina et al [5,26] and others [53,54], have shown that nascent mlRNA in the nucleus is already associated with proteins. Much of these ribonucleoprotein complexes, which exist also in HeLa cells [12,55], remain in the nucleus [52, 561.…”