Ribosome biosynthesis in Tetrahymena thermophila. III. Regulation of ribosomal RNA degradation in growing and growth arrested cells

Abstract

We have measured the turnover rate of ribosomal RNA in exponentially growing Tetrahymena thermophila cells, cells entering the plateau phase of growth, and nutrient-deprived (starved) cells. Ribosomal RNA is stable in cells in early log phase growth but it begins to turnover as the cells begin a deceleratory growth phase prior to entering a plateau state. Likewise, rRNA in cells transferred from early log phase growth to a starvation medium begins to be degraded immediately upon starvation. In both cases the degradation of rRNA exhibits biphasic kinetics. A rapid initial exponential degradation with a half time of nine and one-half hours lasting for six hours is followed by a slower exponential degradation with a half-life of 35 hours. When starved cells are transferred to fresh growth medium turnover of rRNA ceases. The evidence presented suggests that the alteration in degradation rate is a regulated process which is most likely independent of the cell cycle.

Characterization of a cycloheximide-resistant Tetrahymena thermophila mutant which also displays altered growth properties.

“…This change in turnover rate and its cessation at refeeding correlated with all the other changes in ribosome structure and function noted above that we presumed to be the result of a change in the conformational state of the ribosome (10). Figure 9 shows the kinetics of degradation of rRNA in newly starved strain X-8 cells and compares it with the data presented previously for wild-type cells (22). Although the slow turnover rates were the same for the two strains, there was clear-cut difference between the initial rates of rRNA degradation.…”

“…We found a final dissimilarity between the two strains that could be interpreted as resulting from a physical difference in ribosome structure: the turnover rate of rRNA in mature ribosomes of starved cells. rRNA does not turn over in cells in early exponential phase, but after cells are transferred to starvation conditions cellular degradation of rRNA is induced (22). This degradation is biphasic; there is a rapid initial turnover rate that lasts for about 6 h, after which a slower rate ensues.…”

“…The details for rRNA degradation rate measurement have been described (22). In determining the degradation rate, we measured the loss of TCA-insoluble counts in total radioactive cellular RNA.…”

“…This was possible because, after cells had been labeled for two generations in growth medium containing [3H]uridine, followed by two to three generations of growth in nonradioactive medium, 90%o of the TCA-insoluble counts in the cell were in the rRNA, and none were nonalkaline labile. Furthermore, when cells are washed into starvation medium and turnover of RNA is induced, the percentage of RNA remaining TCA insoluble as rRNA remains constant for the first 24 h of starvation (22).…”

Phosphorylation of a 40S ribosomal subunit protein in Tetrahymena

“…In Tetrahymena cells growing in a nutritionally adequate medium, ribosomes are stable throughout the log-phase growth, but begin to turnover as the cells enter maximum stationary phase [9]; therefore, it was of interest to compare the in vivo stability of ribosomes in the different stages of culture growth with the status of the small ribosomal subunit protein S7. For this purpose, cells from a culture growing in PPY medium were harvested in early, mid and late log, deceleration and stationary phases; 80s ribosomes were isolated and their proteins analyzed by two-dimensional gel electrophoresis.…”