The control of ribosomal RNA transcription in lymphocytes. Evidence that the rate of chain elongation is the limiting factor.

During activation of lymphocytes by phytohaemagglutinin, the nuclear activity of RNA polymerase I increases with no proportional increase in the amount of catalytic efficiency of the enzyme in the cell. The mechanism by which rRNA transcription in lymphocytes is modified by phytohaemagglutinin stimulation was investigated. The following results were obtained. (a) In resting lymphocytes all RNA polymerase II molecules are bound to the template while a pool of excess free RNA polymerase I, not engaged in transcription, can be detected by its ability to transcribe added poly[d(A-T)]. (b) Although the free RNA polymerase I activity increases twofold to threefold during phytohaemagglutinin stimulation, there is no evidence that the free enzymes ever become engaged in transcription. (c) Most of the RNA chains in elongation in nuclei from resting lymphocytes are being elongated by class II RNA polymerase and their rate of elongation is much higher than that of other RNA species. (d) The same number of pre-rRNA chains are in the process of being elongated in nuclei from resting and stimulated lymphocytes. However, the rate of elongation of pre-rRNA, which is slow relative to the average in resting lymphocytes, increases twofold to threefold within 6 h of phytohaemagglutinin stimulation and rises to sixfold by 19 h. These results suggest that the control of rRNA transcription in phytohaemagglutinin-stimulated lymphocytes lies in the elongation step of transcription rather than in initiation, and that little or no additional rRNA template is transcribed in phytohaemagglutinin-stimulated lymphocytes.