Physiological studies of tryptophan transport and tryptophanase operon induction in Escherichia coli

Abstract

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 eiiminate 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 a 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. Escherichia coli uses several mechanisms to regulate the expression of its tryptophan (trp) operon and control the rate of tryptophan biosynthesis. The most important of these are repression, transcription attenuation, and feedback inhibition (29). Their combined action permits the bacterium to vary the rate of tryptophan production over a several thousand-fold range. Since tryptophan is costly to produce, efficient shutdown of synthesis is advantageous to the bacterium whenever the amino acid is present in its environment. Consistent with this conclusion, most organisms that feed on other organisms have lost the capacity to synthesize tryptophan.

State-dependent Conformations of the Translocation Pathway in the Tyrosine Transporter Tyt1, a Novel Neurotransmitter:Sodium Symporter from Fusobacterium nucleatum

“…Expression of tyt1 from the inducible T5 promoter in a pQE60 derivative revealed only marginal tyrosine accumulation above background (data not shown). To generate a suitable host strain for detailed kinetic and biochemical studies, we used E. coli CY15212, a strain already deficient in the three aromatic amino acid uptake systems, Mtr, AroP, and TnaB (25), as the parental strain for the subsequent disruption of the tyrP and pheP genes. As shown in Fig.…”

Regulation of the AcrAB multidrug efflux pump in Salmonella enterica serovar Typhimurium in response to indole and paraquat

“…Thus, it is hypothesized that indole binds to the C-terminal domain of RamR and derepresses the transcriptional activity of ramA by causing detachment from the binding site of RamR. In E. coli, indole is produced from tryptophan by tryptophanase and is excreted from the cell (Yanofsky et al, 1991). Salmonella does not produce indole because it lacks the tnaA gene that encodes tryptophanase (McClelland et al, 2001); however, indole is found in various internal human environments, especially in the intestine (Sonnenwirth, 1980).…”

“…Indole, a bacterial metabolite (Yanofsky et al, 1991), is produced from tryptophan by tryptophanase (TnaA) and excreted outside the cell. Indole is a biological oxidant for bacteria, and it induces antioxidant proteins in E. coli and Brevibacterium flavum (Garbe et al, 2000).…”

Role of Indole Production on Virulence of Vibrio cholerae Using Galleria mellonella Larvae Model

“…Indole is produced from tryptophan by tryptophanase (encoded by tnaA) and its production is regulated by the tryptophan (trpABCDE) and tna operons (tnaCAB) [6][7][8]. The level of tryptophan controls transcription-terminating factor (Rho) that affects tna operon and indole production [6,8]. In the low tryptophan level, trp operon is elevated and tna operon is repressed, as Rho occurs in this operon resulting in low indole production.…”

“…Although indole has been reported in both Gram-negative and Gram-positive bacteria, biosynthesis of indole has been studied mostly in Escherichia coli. Indole is produced from tryptophan by tryptophanase (encoded by tnaA) and its production is regulated by the tryptophan (trpABCDE) and tna operons (tnaCAB) [6][7][8]. The level of tryptophan controls transcription-terminating factor (Rho) that affects tna operon and indole production [6,8].…”