Regulation of ribosome synthesis in Tetrahymena pyriformis. 3. Analysis by translation in vitro of RNA isolated during nutritional shift-down and nutritional shift-up.

We have measured the levels of translatable total mRNA and ribosomal protein (r-protein) mRNAs in Tetrahymena pyriformis during nutritional shifts. After 15 min of starvation total mRNA is reduced 2-fold, and after 24 h 7.5-fold, relative to exponentially growing cells. Upon refeeding total mRNA increases rapidly reaching the level of exponentially growing cells after 2.5 h. The levels of the individual r-protein mRNAs are coordinately regulated throughout a starvation-refeeding cycle. The relative levels of r-protein mRNAs remain virtually unchanged during the first hour of starvation and then decrease gradually to 30% of the relative levels in exponentially growing cells. Following refeeding the relative levels of r-protein mRNAs increase 6-fold. Taking into account the changes in whole cell RNA, we have calculated that long-time-starved Tetrahymena cells contain only 4%, whereas cells after 3 h of refeeding contain 200% of the amount of r-protein mRNA in exponentially growing cells. The amount of r-protein mRNA thus increases 50-fold during the first 3 h of refeeding. A comparison between the relative levels of r-protein mRNAs and the relative rate of r-protein synthesis in vivo indicates that Tetrahymena employs a combination of control of translation and control of the level of r-protein mRNAs to ensure a rapid reduction in the rate of r-protein synthesis during the early period of starvation. In this period translation of r-protein mRNAs is preferentially inhibited. During refeeding the increase in the rate of r-protein synthesis parallels the increase in the abundance of r-protein mRNAs.