Springer W, Grimminger H, Lingens F
J Bacteriol. 1972 Oct;112(1):259-63. doi: 10.1128/jb.112.1.259-263.1972.
Isoleucine-deficient mutants of Salmonella typhimurium were isolated. Three groups of mutants can be discerned by their nutritional requirements and enzyme patterns. (i) Mutants which grow with isoleucine alone are devoid of biosynthetic threonine deaminase (TD). (ii) Mutants growing with isoleucine and valine are devoid of transaminase B. (iii) Mutants growing with either isoleucine or threonine have normal levels of TD. However, the sensitivity of this enzyme to feedback inhibition by isoleucine is greatly enhanced. The inhibitory effect of isoleucine can be counterbalanced by high concentrations of threonine. These results indicate that the production of isoleucine in the mutants is restricted to a low level not sufficient to support the growth of the cells. This hypothesis is confirmed by studies with revertants of an isoleucine-threonine mutant. In nine revertants, wild-type properties of TD have been restored. In four revertants, the hypersensitivity of TD is unchanged, but the strains produce a greatly enhanced quantity of threonine, which is excreted into the culture medium. It follows, that hypersensitivity of TD to inhibition by isoleucine is the cause of the nutritional requirement of isoleucine-threonine mutants.
鼠伤寒沙门氏菌的异亮氨酸缺陷型突变体被分离出来。根据营养需求和酶模式可区分出三组突变体。（i）仅在异亮氨酸存在时生长的突变体缺乏生物合成苏氨酸脱氨酶（TD）。（ii）在异亮氨酸和缬氨酸存在时生长的突变体缺乏转氨酶B。（iii）在异亮氨酸或苏氨酸存在时生长的突变体具有正常水平的TD。然而，这种酶对异亮氨酸反馈抑制的敏感性大大增强。异亮氨酸的抑制作用可被高浓度的苏氨酸抵消。这些结果表明，突变体中异亮氨酸的产生被限制在不足以支持细胞生长的低水平。用异亮氨酸 - 苏氨酸突变体的回复突变体进行的研究证实了这一假设。在九个回复突变体中，TD的野生型特性得以恢复。在四个回复突变体中，TD的超敏感性未改变，但菌株产生大量增强的苏氨酸，并分泌到培养基中。由此可见，TD对异亮氨酸抑制的超敏感性是异亮氨酸 - 苏氨酸突变体营养需求的原因。
