Yanofsky C, Horn V, Gollnick P
Department of Biological Sciences, Stanford University, California 94305-5020.
J Bacteriol. 1991 Oct;173(19):6009-17. doi: 10.1128/jb.173.19.6009-6017.1991.
Escherichia coli forms three permeases that can transport the amino acid tryptophan: Mtr, AroP, and TnaB. The structural genes for these permeases reside in separate operons that are subject to different mechanisms of regulation. We have exploited the fact that the tryptophanase (tna) operon is induced by tryptophan to infer how tryptophan transport is influenced by the growth medium and by mutations that inactivate each of the permease proteins. In an acid-hydrolyzed casein medium, high levels of tryptophan are ordinarily required to obtain maximum tna operon induction. High levels are necessary because much of the added tryptophan is degraded by tryptophanase. An alternate inducer that is poorly cleaved by tryptophanase, 1-methyltryptophan, induces efficiently at low concentrations in both tna+ strains and tna mutants. In an acid-hydrolyzed casein medium, the TnaB permease is most critical for tryptophan uptake; i.e., only mutations in tnaB reduce tryptophanase induction. However, when 1-methyltryptophan replaces tryptophan as the inducer in this medium, mutations in both mtr and tnaB are required to prevent maximum induction. In this medium, AroP does not contribute to tryptophan uptake. However, in a medium lacking phenylalanine and tyrosine the AroP permease is active in tryptophan transport; under these conditions it is necessary to inactivate the three permeases to eliminate tna operon induction. The Mtr permease is principally responsible for transporting indole, the degradation product of tryptophan produced by tryptophanase action. The TnaB permease is essential for growth on tryptophan as the sole carbon source. When cells with high levels of tryptophanase are transferred to tryptophan-free growth medium, the expression of the tryptophan (trp) operon is elevated. This observation suggests that the tryptophanase present in these cells degrades some of the synthesized tryptophan, thereby creating a mild tryptophan deficiency. Our studies assign roles to the three permeases in tryptophan transport under different physiological conditions.
Mtr、AroP和TnaB。这些通透酶的结构基因位于不同的操纵子中，受不同调控机制的影响。我们利用色氨酸酶（tna）操纵子由色氨酸诱导这一事实，来推断色氨酸转运如何受生长培养基以及使每种通透酶蛋白失活的突变的影响。在酸水解酪蛋白培养基中，通常需要高水平的色氨酸才能使tna操纵子达到最大诱导。需要高水平是因为添加的大部分色氨酸会被色氨酸酶降解。一种被色氨酸酶切割效率低的替代诱导剂1-甲基色氨酸，在tna⁺菌株和tna突变体中低浓度时就能有效诱导。在酸水解酪蛋白培养基中，TnaB通透酶对色氨酸摄取最为关键；即只有tnaB中的突变会降低色氨酸酶诱导。然而，当1-甲基色氨酸在这种培养基中替代色氨酸作为诱导剂时，mtr和tnaB中的突变都需要才能阻止最大诱导。在这种培养基中，AroP对色氨酸摄取没有作用。然而，在缺乏苯丙氨酸和酪氨酸的培养基中，AroP通透酶在色氨酸转运中具有活性；在这些条件下，必须使三种通透酶失活才能消除tna操纵子诱导。Mtr通透酶主要负责转运吲哚，吲哚是色氨酸酶作用产生的色氨酸降解产物。TnaB通透酶对于以色氨酸作为唯一碳源的生长至关重要。当具有高水平色氨酸酶的细胞转移到不含色氨酸的生长培养基中时，色氨酸（trp）操纵子的表达会升高。这一观察结果表明，这些细胞中存在的色氨酸酶会降解一些合成的色氨酸，从而造成轻度色氨酸缺乏。我们的研究确定了这三种通透酶在不同生理条件下色氨酸转运中的作用。
