Regulation of the biosynthesis of valyl-tRNA synthetase in yeast

Abstract

When a wild type yeast is grown first in the presence of valine, then transferred to a medium without valine, there is a 2-fold increase in the specific activity of valyl-tRNA synthetase after this shift-down. The addition of cycloheximide at the time of the transfer prevents this increase in specific activity. If valine is added again 2.5 h after the shift in one half of the culture, there is a decrease in specific activity of the enzyme, while in the other half, where no valine has been added, the increase in specific activity is maintained. Addition of valine to exponentially growing cells is followed by a 25–30% decrease in valyl-tRNA synthetase activity. The addition, at the time of the shift-down, of α-aminobutyric acid, an analog of valine which is activated by valyl-tRNA synthetase but not charged on tRNA Val, does not interfere with the increase in specific activity of the enzyme caused by the removal of valine from the medium. But the addition of α-amino-β-chlorobutyric acid, an analog which is activated and charged on tRNA Val, prevents the increase in specific activity of valyl-tRNA synthetase after the shift-sdown. It is possible to grow yeast in the presence of valine and 2H 2O and to perform the shift to a medium containing H 2O but no valine. Under these conditions, the enzymes synthesized before the shift are heavier than those made after the shift and they can be separated upon centrifugation in CsCl. These experiments have shown that removal of valine from the medium causes a specific increase in the amounts of newly synthesized valyl-tRNA synthetase. Of the 2 analogs, only α-amino-β-chlorobutyric acid (which can be charged on tRNA Val) prevents this increase, which suggests that valyl-tRNA is a key-component of the repression mechanism.