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From previous studies in other laboratories, Earl was aware that the amount of enzymes in cells was generally affected by the nutritional state of the organism. Taking this information as a clue, Earl and co-workers set up experiments for the controlled starvation of E. coli , by cutting off the source of nitrogen or carbon during its growth in culture media; and they examined the amounts of fifteen enzymes that were extracted from these starved bacteria. The results were mixed: some enzymes showed an increase in their amounts during starvation; others a decrease; and still others no change. Earl then focused on the enzymes whose level of concentration dropped substantially. Among them was glutamine synthetase, which once again became a central research material in Earl's laboratory.

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