The movement of ribonucleic acid (RNA) from nucleus to cytoplasm has been studied, by autoradiographic techniques, in cells of the human amnion grown in tissue culture.Cells were exposed to cytidine-H 3 for 1 hour after which time only the RNA of the nuclei was labelled. After this 1 hour exposure the cells were placed in a medium containing an excess amount of unlabelled cytidine. Periodically, cells from this medium were fixed. Autoradiographs showed that there was a progressive movement of the label from nucleus to cytoplasm, such that after 24 hours essentially all the label was in the RNA of the cytoplasm.A study of the incorporation of the cytidine-H * in deoxyribonucleic acid (DNA), in the same population of cells at the same times, indicated that the presence of excess amounts of unlabelled cytidine almost instantaneously inhibited further utilization of cytidine-H 3.It is concluded that RNA moves from nucleus to cytoplasm as a complex polynucleotide structure.The problem of whether or not cellular ribonucleic acid (RNA) transmits information from gene to cytoplasmic protein continues to be of considerable interest and the stimulus to much research. As part of this problem, many workers have been interested in determining whether or not nuclear R N A (nRNA), possibly the product of chromosomal activity, is a precursor of cytoplasmic R N A (cRNA), which would express genetic information through the synthesis of proteins in the cytoplasm. Brachet has admirably discussed this problem and his book (3) should be consulted for a review of the pertinent literature up to 1957.The work reviewed by Brachet indicates that there is some controversy with regard to whether

There is increasing cytological evidence that a substantial portion of cytoplasmic ribonucleic acid (RNA) is synthesized in the nucleus (1-4). In two of these papers (3, 4) as well as in earlier work (see pp. 133-135, reference 5) the evidence suggested that the nucleolus was the site (within the nucleus) where RNA synthesis was almost completely localized.Since there are very few nucleoli per nucleus, in most cell types, the observation that the incorporation of RNA precursors into nucleolar RNA is more rapid and occurs much earlier than in other areas casts some doubt on the hypothesis that RNA is a carrier of information from gene to cytoplasm. That is to say, it is difficult to visualize how information from many chromosomal loci could be imparted to RNA synthesized at only a few loci represented by the nucleoli. The hypothesis might still prove valid, however, if one were to assume that RNA was synthesized at many chromosomal loci and was very rapidly transported to the nucleoli where some other activity took place, at a much slower rate. (This other activity might be the conjugation of RNA with protein). Such a hypothesis has been suggested by Woods and Taylor (3).If the latter assumption is valid, then one might find that following a very brief exposure to a radioactive RNA precursor, only non-nudeolar nuclear (or chromosomal) RNA would be labelled. The experiments herein described were concerned with testing this hypothesis. ExperimentalThe methods and materials employed were essentially identical with those described in a more extensive paper (4). Briefly, the procedure was as follows. Human amnion cells of an established cell line (6) were exposed to a normal growth medium containing 30 #c./ml. of tritiated cytidine, (specific activity 360 mc./mi) for * Supported by funds from the University of California Cancer Research Funds and the Brinton Fund for Cancer Research, administered by Dr. David A. Wood.;~ Address after July 1, 1959: Division of Biology, University of Pennsylvania, Philadelphia. § Received for publication, May 5, 1959. periods of 2, 5, and 10 minutes before fixation. Some cells were treated with ribonuclease (0.4 rag. Worthington crystalline RNase/ml. at pH 6.7, incubated at 37°C. for 21/~ hours) and other cells were left untreated. The cells were then covered with autoradiographlc stripping film in the dark, stored for 30 days, and developed in the usual manner. The specimens were then examined by phase contrast and brightfield microscopy. RESULTSExamination of the specimens revealed very slight labelling of the cells after the 2 minute exposure to tritiated cytidine. The 5 minute sample showed significant nuclear, but no cytoplasmic labelling. Contrary to earlier observations, however, there was no very marked labelling over the nucleoli (Fig. 1). On the other hand, at 10 minutes there was already a marked degree of labelling over the nucleoli (Fig. 2). In order to obtain a quantitative measure of the extent of the activity over the nucleoli as contrasted to the rest of the nucleus...

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