Mutation in the structural gene for seryl-transfer ribonucleic acid synthetase of Escherichia coli which affects formation of its gene product at high temperature.

The rate of formation of seryl-transfer ribonucleic acid (tRNA) synthetase activity was temperature dependent in a temperature-sensitive mutant of Escherichia coli (K28) with an altered seryl-tRNA synthetase structural gene and in a class of spontaneous revertants derived from it. These revertants, which were selected by their ability to grow at 45 degrees C, had high levels of the thermolabile enzyme. The rate of formation of seryl-tRNA synthetase activity in the mutant and in the revertants fell from 100% to near zero with a 4 degrees C temperature range from 40 to 44 degrees C. The temperature-dependent rate of formation of seryl-tRNA synthetase activity was reversible. Dropping the temperature from 44 to 37 degrees C resulted, after a 2- to 3-min delay, in a resumption of the initial rate of formation of enzyme activity. The results could not be accounted for by in vivo or in vitro degradation of the active enzyme. Addition of rifampin just before the temperature shift down from 44 to 37 degrees C inhibited the appearance of seryl-tRNA synthetase activity at the lower temperature. Explanations which might account for these phenomena are proposed.