RAPIDLY LABELED CYTOPLASMIC RNA IN NORMAL AND PHYTOHAEMAGGLUTININ-STIMULATED HUMAN LYMPHOCYTES

1. 1. Phytohaemagglutinin caused a significant stimulation of the incorporation of [3H]uridine into RNA within 30 min of its addition to lymphocytes. While more than 90% of the radioactive RNA labeled in this period was in the nucleus, the stimulation of [3H]uridine incorporation into RNA located in the cytoplasm was greater than the stimulation of incorporation into whole cell RNA. This rapid stimulation of the labeling of cytoplasmic RNA by phytohaemagglutinin might be important for the changes in lymphocyte protein synthesis that began at this time. 2. 2. Analysis by sucrose gradient centrifugation of the cytoplasmic RNA labeled in a 30-min pulse with [3H]uridine showed that 4-S RNA was the predominant labeled species. However, gel filtration on Sephadex G-100 showed that this labeled RNA contained several components and that little of it was tRNA. Phytohaemagglutinin did not alter the elution pattern of the radioactive cytoplasmic RNA from Sephadex G-100 but stimulated the labeling of all its components to about the same extent. 3. 3. The predominant rapidly labeled cytoplasmic RNA species was retarded by Sephadex G-100 but eluted in a polydisperse fashion ahead of tRNA. This species was unstable to nuclease digestion and was not methylated. Its disappearance during an actinomycin chase was accompanied by the appearance of labeled tRNA. It did not seem to be attached to cytoplasmic particles, and it hybridized to lymphocyte DNA less readily than did nuclear polydisperse RNA. These properties are compatible with it being an unmethylated cytoplasmic precursor of tRNA. 4. 4. There was no evidence for an increase in the labeling of mature tRNA 30 min after the addition of phytohaemagglutinin, but such an increase was apparent after 90 min. After incubation of lymphocytes with phytohaemagglutinin for 20-24 h, the rate of tRNA synthesis was greatly stimulated. If the material discussed above is indeed a precursor to tRNA, the rate of its conversion to mature tRNA was also accelerated at this time. © 1969.