| Abstract: | Dilution of a stationary phase culture of Scarlet Rose results in an increased rate of protein synthesis. This study compares the time course of this increase with the changes in polyribosome content and the levels of adenine and guanine nucleotides. During the first two hours after dilution, protein synthesis increases 2- to 3-fold; much of the large monoribosome pool that characterizes the stationary state disappears and a steady state situation is reached in which 70% of the ribosomes are in polyribosomes. Between two and eight hours, there is no further change in polyribosome content although the rate of protein synthesis increases an additional 2- to 3-fold. During this initial 8-hour period there is little change in the levels of ATP and GTP. An explanation consistent with these observations is that the initial activation (within the first 2 hours), characterized by the monoribosome to polysome transition, is at the level of a component(s) of the initiation system, and that between two and eight hours, since neither mRNA availability nor energy level are primary determinants, protein synthesis is augmented by the activation of a translational component, perhaps an elongation factor. After 24 hours, there is a proliferative phase characterized by the onset of ribosome accumulation. By day 5, maximum ribosome levels, 5-fold that of 24-hour cells, are reached, but the rate of protein synthesis increases only 2.5-fold during this period. The lack of quantitative coincidence between the changes in polyribosome content and the rates of protein synthesis again suggests that factors other than mRNA availability are involved in determining the overall rate of protein synthesis. Finally at days 6–8, while the growth of the culture is still in the exponential phase, the rate of protein synthesis per unit fresh weight drops markedly concomitant with a decline in ribosome content. At days 11–12, the monoribosome to polysome ratio begins to change with the monoribosome pool increasing. Presence of either actinomycin D or cordycepin inhibits increased protein synthesis in direct relation to the ability of these compounds to inhibit RNA synthesis. This suggests that the protein synthetic processes occurring after dilution require either the synthesis of the mRNA that is being translated or of an RNA functioning in a closely linked reaction. |
