Proof that certain RNAs shuttle non-randomly between cytoplasm and nucleus

When a nucleus containing 3H-RNA is grafted into an unlabeled cell the recipient cell nucleus acquires approx. 10 times the concentration of 3H-RNA acquired by the cytoplasm, which suggests that some RNA molecules shuttle between cytoplasm and nucleus against a concentration gradient. We report here on three lines of evidence that show that the acquisition of radioactive RNA by the recipient cell nucleus is unlikely to result from the synthesis within the recipient nucleus of new RNA from simple radioactive precursors such as might be produced by the breakdown of 3H-RNA. 1.1. The size of the labeled precursor pool seems inadequate to account for the labeling of host nuclear RNA through new transcriptions.2.2. The 3H-RNA destined for one-way passage from nucleus to cytoplasm can be diluted out by sequential passage of a labeled nucleus through unlabeled cytoplasms resulting in a nuclear enrichment of the putative shuttling 3H-RNA. The result of such an operational sequence is that the last unlabeled recipient cell nucleus acquires 100 or more times the concentration of 3H-RNA found in the cytoplasm, supporting the view that there has been an enrichment for labeled shuttling RNAs.3.3. Electron microscopic radioautography reveals that the 3H-RNA acquired by a recipient cell nucleus is almost entirely non-nucleolar, whereas the 3H-RNA in a nucleus directly labeled with simple radioactive precursors is largely within nucleoli. This clearly implies that the labeled material that moves from one nucleus to another via the cytoplasm is something more than a simple transcription precursor.