Abstract
The ribosomal activity in human peripheral blood lymphocytes stimulated by concanavalin A has been characterized during the passage of cells from the resting (G~0~) state, through maximum mitotic activity, and into a secondary resting state. The total ribosome content increased progressively after the addition of concanavalin A and after 72 hours reached a maximal 10βfold increase. Lymphocytes cultured for 120 hours returned to a low metabolic activity, but still contained four times more ribosomes per cell than unstimulated lymphocytes. During the first 24 hours free ribosomes, which were originally 64% of resting cell ribosomes, decreased to 27% of total particles. Thereafter they reaccumulated until, at 120 hours, free ribosomes again comprised 61% of the total ultraviolet absorbance and the total polysome profile resembled that for resting cells, despite the higher ribosome content.
Analysis of the rate of protein synthesis and the corresponding quantity of polysomes at different times in the growth sequence revealed that protein synthesizing efficiency of polysomes was initially low and rose for the first 48 hours of culture. After 48 hours, polysomes with low protein synthesizing efficiency reappeared. Following pulse labeling of rRNA with [^3^H]uridine, the redistribution of newly synthesized ribosomal particles was followed. A decreased flow of ribosomes through polysomes was evident at 48 hours and thereafter. Diminution in polysome efficiency, reduced flow of particles through polysomes, and accumulation of free ribosomes thus characterize the cessation of growth which occurs as a physiological event in Con Aβstimulated lymphocyte cultures. Nevertheless, these changes begin at or before the peak of RNA, protein and DNA synthetic activity, showing that the biochemical mechanisms related to cessation of cell growth begin to function during the height of cell growth in a system under physiological control.