Degradation of 28s Rna Late in Ribosomal Rna Maturation in Nongrowing Lymphocytes and Its Reversal after Growth Stimulation

In physiologically nongrowing lymphocytes, the rate of entry of new ribosomal RNA (rRNA) molecules to the cytoplasm is only half the rate at which rRNA is synthesized in the nucleus, although there is no net nuclear accumulation of rRNA (6) . When total cell RNA is extracted from intact lymphocytes after moderately short labeling periods, the amount of radioactivity appearing in 18S rRNA is only half that anticipated from the labeling observed in 32S + 28S molecules . However, new 28S and 18S molecules appear in the cytoplasm in the expected equimolar ratio (2-4, 6) . After growth stimulation with phytohemagglutinin (PHA), both of the above disparities are rapidly eliminated . We attributed these findings to the degradation or "wastage" of half of the newly synthesized rRNA molecules in resting lymphocytes. Elimination of such wastage was proposed as an important early step in the shift from the resting to the growing condition (2-4, 6). Evidence indicated that the aberrant (32S + 28S) :18S labeling ratio noted above was due to synthesis and rapid degradation of new 18S RNA while coordinately synthesized 32S + 28S molecules still survived (2-4, 6) . Since no excess of labeled 28S rRNA appeared in the cytoplasm, it was reasoned that the extra 32S + 28S molecules were restricted to the nucleus . Maintenance of cellular integrity would not permit continued accumulation of excess 32S + 28S RNA in the nucleus, and so it was predicted that degradation of these molecules would also be seen, but only after some delay . Failure to confirm this prediction would cast doubt on the wastage interpretation of our data . In the experiments to be described, delayed degradation of 32S and 28S molecules was demonstrated, together with its reversal after PHA stimulation .