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

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Abstract
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‐protien 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‐protien mRNAs and the relative rate of r‐protein synthesis in vivo indicates that Tetrahymena employs a eombiantion 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 peroid 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‐protine mRNAs.