Isolation and identification of mutants constitutive for aspartokinase III synthesis in Escherichia coli K 12.

We devised a procedure in order to isolate, in Escherichia coli, constitutive mutants for aspartokinase III synthesis, the first enzyme of the lysine regulon. It consists of the introduction of a limiting step in lysine biosynthesis, by the use of the partial suppression of a nonsense mutation. For the first time we could isolate many constitutive mutants. Their characteristics (cotransduction with the lysC structural gene; no effect on the synthesis of other enzymes of the regulon; cis-dominance) lead to classify these mutations as operator-type. The fact that no repressor mutations could be isolated is discussed.