Ribonucleic Acid Metabolism in Lymphocytes Stimulated by Phytohemagglutinin: II. RAPIDLY SYNTHESIZED RIBONUCLEIC ACID AND THE PRODUCTION OF RIBOSOMAL RIBONUCLEIC ACID

Abstract

Abstract Human peripheral lymphocytes are stimulated to change, in vitro, from a resting state to one of progressive enlargement, culminating in mitosis, by treatment with Phaseolus vulgaris phytohemagglutinin. The sedimentation characteristics and fate of rapidly synthesized RNA have been studied in this system after 20 hours of stimulation with phytohemagglutinin, a time when RNA synthesis has been accelerated but DNA synthesis has not begun. The following findings are reported. 1. RNA synthesized during any short interval was found to be largely polydisperse in sedimentation behavior, with s values predominantly greater than 28 S. This material did not serve as a macromolecular precursor to ribosomal RNA or to other discretely sedimenting RNA species. Base composition determinations, by 32P incorporation, indicated that this material was neither ribosome-like nor DNA-like, since it had a high uridylate and low adenylate content. Estimates of the half-life of the polydisperse RNA gave a range of ½ to 2 hours. 2. The sequence of events in the production of ribosomal RNA was examined in detail in the lymphocyte system and was found to be essentially similar to that reported for HeLa cells: a 45 S precursor is converted, during a period of 15 min, to 32 S and 18 S molecules. The 32 S molecule is then converted, during the next 1 to 2 hours, to the 28 S ribosomal RNA. 3. Lymphocytes were found to be relatively resistant to the effects of actinomycin D, requiring longer times of exposure, and larger doses, to produce the inhibitory effects on RNA synthesis which have been reported with other cell systems.