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

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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.