Evolving patterns of RNA metabolism during transition from resting state to active growth in lymphocytes stimulated by phytohemagglutinin.

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Abstract
Stimulation of lymphocytes to enlarge and divide by treatment with PHA [phytohemagglutinin] provides an opportunity to study the changes in RNA metabolism accompanying the conversion of a human cell population from the resting to the growing state. Sucrose gradient sedimentation studies demonstrated an evolving pattern of RNA synthesis in PHA-stimulated cultures of purified lymphocytes. In the resting state, cells produced small amounts of rRNA. An early transition period occurred after treatment with PHA, which was marked by an abrupt increase in the rate of synthesis of polydisperse and 4S RNA. The synthesis of rRNA was increased to a much smaller extent at that time. During a later transition period, beginning 1-6 hr. after treatment with PHA, increased rRNA synthesis was more clearly shown. This increase roughly paralleled the emergence of peaks of pulse-labeling RNA at 45S and 30S. Simultaneously, unstable pulse-labeling RNA began to appear in increasingly greater proportions. Finally, in a phase of active growth (from the 60th hr. to the 7th day), there was prominent synthesis of rRNA and its apparent precursors. The greatest proportion of incorporated label was found in material which was not conserved following a chase with actinomycln D and unlabeled uridine.