Ribosomal RNA production in cultured lymphocytes appeared to be regulated by the rate of 45S rRNA precursor transcription and also by the rate of 45S processing to the 28S and 18S components found in mature ribosomes. Cultures of resting lymphocytes restricted ribosome formation by maintaining both transcription and processing of 45S RNA at low levels. Still about 50 per cent of the RNA finally processed to 28S lacked corresponding 18S components as a result of selective degradation in resting lymphocytes. After PHA treatment, the rates of 45S rRNA precursor transcription and cleavage to 28S products, detectably increased within one hour, rose to a maximum at 48 hours. However, conservation of 18S components reached maximal efficiency between one to five hours after PHA treatment. By 48 hours, at the height of the growth response, PHA-stimulated cultures once again began to selectively degrade 18S RNA. At 168 hours, after the proliferative process had ceased, 45S processing slowed considerably despite a brisk rate of 45S RNA transcription —approximately 50-75 per cent of maximum.

Thus, the initiation of lymphocyte growth and the maintenance of the growing condition may depend on the rate of ribosome biosynthesis which, in turn, may be regulated by the efficiency of 45S rRNA precursor utilization for ribosome assembly.

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