Effects of changes in growth rate on the levels of several aminoacyl-tRNA synthetases in yeast.

When the growth rate of yeast cells is decreased (for instance by transferring the cells from a rich medium to a poor one, or when the cells enter the stationary phase, or when growth is inhibited by cycloheximide, or when valine is removed from the medium supporting growth of a valine-requiring mutant, or when a thermosensitive mutant is shifted to the non-permissive temperature) there is a decrease in the levels of the four aminoacyl-tRNA synthetases tested. Conversely, an increase in the growth rate is accompanied by an increase in the levels of the four enzymes. But when the growth rate is slowed down by decreasing the temperature of the medium, no effect on the levels of the aminoacyl-tRNA synthetases is observed. These results are consistent with the concept of "metabolic regulation" proposed by Parker and neidhardt.