Regulation of protein synthesis in Tetrahymena. Quantitative estimates of the parameters determining the rates of protein synthesis in growing, starved, and starved-deciliated cells.

The calculated rate of protein synthesis for growing Tetrahymena is 360 pg/h, whereas starved cells synthesize only about 3 pg of protein/h. Within 50 min after deciliation of starved cells, the rate of protein synthesis increases to about 60 pg/h. The major mechanism to accomplish these large and rapid changes in the rate of bulk protein synthesis involves regulation of the number of messages loaded on polysomes. Logarithmically growing cells contain about 3.2 X 10(7) mRNA molecules/cell, of which approximately 60% is loaded on polysomes. Starved cells contain about 0.8 X 10(7) messages and the percentage of messages loaded is reduced to 4%. Thus, the number of loaded messages is approximately 60-fold lower in starved cells than in growing cells, although the total message content of cells in these two physiological states differs by only a factor of 4. After deciliation of starved cells, message loading increases about 10-fold. It seems likely that much of the message loaded after deciliation is derived from the large pool of nonpolysomal message in starved cells. Although large differences in message loading exist for growing, starved, and starved-deciliated cells, measurements of the rate of polypeptide elongation and the rate of message initiation indicate the translational efficiency of loaded messages (pg of protein synthesized per pg of message/unit time) is very similar under all conditions.