Wild J, Hennig J, Lobocka M, Walczak W, Kłopotowski T
Mol Gen Genet. 1985;198(2):315-22. doi: 10.1007/BF00383013.
Evidence is presented that alanine racemase activity in E. coli K12 is due to two distinct gene products. The predominant isozyme is inducible by either alanine stereoisomer and repressible by glucose. The gene dadX coding for its structure is located by the dadA gene determining the structure of D-amino acid dehydrogenase. The regulatory site for the expression of both genes, dadR, is located on the other side of dadA. The orientation of the dad operon established by multiple-point crosses and deletion mapping is as follows: fadR ...dadRAX ...hemA. The dadX alanine racemase activity is unusually refractory to changes of incubation temperature. It differs strikingly from that of the other isozyme, probably the product of the alr gene. The latter isozyme shows a typical dependence upon incubation temperature. The synthesis of alr alanine racemase is constitutive in respect of both alanine and glucose. In dadX mutants, in which alanine racemase activity equals only 15% of that in wild-type cells grown in the absence of an inducer or catabolite repressor, the dad operon cannot be induced by D-alanine. We presume, therefore, that L-alanine is involved more directly than D-alanine in dad operon regulation.
有证据表明，大肠杆菌K12中的丙氨酸消旋酶活性归因于两种不同的基因产物。主要的同工酶可被任何一种丙氨酸立体异构体诱导，并可被葡萄糖抑制。编码其结构的基因dadX位于决定D-氨基酸脱氢酶结构的dadA基因附近。这两个基因（dadR）表达的调控位点位于dadA的另一侧。通过多点杂交和缺失图谱确定的dad操纵子的方向如下：fadR...dadRAX...hemA。dadX丙氨酸消旋酶活性对孵育温度的变化异常不敏感。它与另一种同工酶（可能是alr基因的产物）有显著差异。后一种同工酶表现出对孵育温度的典型依赖性。alr丙氨酸消旋酶的合成在丙氨酸和葡萄糖方面都是组成型的。在dadX突变体中，丙氨酸消旋酶活性仅为在无诱导剂或分解代谢物阻遏物的情况下生长的野生型细胞的15%，dad操纵子不能被D-丙氨酸诱导。因此，我们推测L-丙氨酸比D-丙氨酸更直接地参与dad操纵子的调控。
