Ribosomal Ribonucleic Acid Wastage in Resting and Growing Lymphocytes

The synthesis of 28S and 18S ribosomal RNA in human lymphocytes was investigated under resting conditions and at various times during cell growth induced by phytohemagglutinin. The currently accepted model for ribosomal RNA production in animal cells holds that both 28 S and 18 S subtypes are synthesized as part of a single 45 S precursor, thus predicting their synthesis in 1:1 molar ratio. Deviations from this ratio are taken to indicate rapid degradation, after synthesis, of one species in excess of the other. With the use of methionine-methyl-3H as a means of labeling newly synthesized rRNA, the following observations were made: 1. In resting lymphocytes about one half of the newly synthesized 18 S rRNA was immediately degraded, confirming a previous report. 2. Upon addition of the growth stimulant, phytohemagglutinin, the degree of rRNA wastage began to diminish almost immediately. The least degradation was observed after 6 to 7 hours of stimulation. 3. After 20 hours of incubation with PHA, rRNA wastage was still low, but had risen to a level significantly above that found at 6 to 7 hours. After 40 hours, at a time when maximal cell growth was in progress, rRNA wastage was almost as high as that in resting cells. 4. Addition of excess phytohemagglutinin during maximal growth did not reverse the wastage of rRNA. However, when the cells were permitted to incubate for 9 days, at which time growth activity was reduced to nearly resting levels, rRNA wastage was again promptly reduced by readdition of phytohemagglutinin. It is concluded that reduction of rRNA wastage is an early event in stimulation of lymphocyte growth, but that rRNA wastage is not incompatible with cell growth under conditions of elevated rRNA synthesis.