The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli

In Escherichia coli K₁₂,

• 1.

  1. A single mutation can lead to the concomitant modification or to the concomitant loss of the two activities, threonine-sensitive β-aspartokinase (ATP: L-aspartate 4-phosphotransferase, EC 2.7.2.4) and threonine-sensitive homoserine dehydrogenase (L-homoserine: NADP⁺ oxidoreductase, EC 1.1.1.3).

• 2.

  2. The two activities cannot be separated and the ratio of the specific activities remains constant throughout a 600-fold purification.

• 3.

  3. The substrates of one of the activities are inhibitors of the otther activity. The observed inhibitions are specific.

• 4.

  4. The threonine-sensitive aspartokinase is protected against thermal inactivation by NADPH, a substrate of the homoserine dehydrogenase.

• 5.

  5. The conclusion is drawn that the two activities under study are carried by a single protein molecule. The apparent molecular mass of this complex protein is changed in some mutants. There is no apparent correlation between the apparent molecular mass and the cooperativity of inhibitor molecules.

• 6.

  6. The significance of these findings is discussed in terms of metabolic regulation and intracellular topology.