Regulation of Glutamine Synthetase in Cultured Chinese Hamster Cells

Glutamine synthetase activity in Chinese hamster cells in tissue culture possesses properties identical with purified liver glutamine synthetase. The cellular level of enzyme is regulated by the concentration of glutamine in the growth media. Induction by removal of glutamine leads to an 8- to 10-fold increase in enzyme activity within 48 hours. Repression is a much more rapid process. Half of the activity is lost within 12 min after adding 1 mm glutamine to the media, and full repression occurs in 8 hours. The decrease in enzyme activity is directly proportional to glutamine for concentrations less than 0.3 mm. Actinomycin D has little effect on induction or repression, indicating that regulation does not occur at the level of messenger RNA synthesis. Cycloheximide blocks induction, indicating that protein synthesis (perhaps of new enzyme) is required for increased enzyme levels. Protein synthesis is not required for repression. We propose that a glutamine-mediated modification of glutamine synthetase is responsible for repression of enzyme activity.