Vapnek D, Greer S
J Bacteriol. 1971 Jun;106(3):983-93. doi: 10.1128/jb.106.3.983-993.1971.
Challenging auxotrophs on metabolites that are precursors of a biosynthetic step involving a mutated enzyme has revealed a new class of suppressor mutations which act by derepressing a minor enzyme activity not normally detected in the wild-type strain. These indirect, partial suppressor mutations which allow isoleucine auxotrophs to grow on homoserine or threonine have been analyzed to determine their effect on enzymes involved in the biosynthesis of these amino acids. It has been found that one class of these suppressor mutations (sprA) leads to the derepression of homoserine kinase, homoserine dehydrogenase, and a minor threonine dehydratase that is not sufficiently active to be detected in the wild-type strain. The gene encoding this second threonine dehydratase activity has been found to be located between the structural genes for homoserine kinase and homoserine dehydrogenase. The results of these experiments indicate that plating of auxotrophs on precursors of a biosynthetic step involving mutated enzymes could prove to be a valuable method for the detection of regulatory mutants as well as a possible tool in studying the evolution of biochemical pathways.
在涉及突变酶的生物合成步骤的前体代谢物上对营养缺陷型进行挑战，揭示了一类新的抑制突变，其作用方式是解除对野生型菌株中通常检测不到的次要酶活性的抑制。这些间接的部分抑制突变使异亮氨酸营养缺陷型能够在高丝氨酸或苏氨酸上生长，已对其进行分析以确定它们对参与这些氨基酸生物合成的酶的影响。已发现这类抑制突变中的一类（sprA）导致高丝氨酸激酶、高丝氨酸脱氢酶以及一种次要的苏氨酸脱水酶的去抑制，这种次要的苏氨酸脱水酶在野生型菌株中活性不足以被检测到。已发现编码这种第二种苏氨酸脱水酶活性的基因位于高丝氨酸激酶和高丝氨酸脱氢酶的结构基因之间。这些实验结果表明，将营养缺陷型接种在涉及突变酶的生物合成步骤的前体上，可能被证明是检测调控突变体的一种有价值的方法，也是研究生化途径进化的一种可能工具。
