Genetic fusions that help define a transcription termination region in Escherichia coli

The trpA gene product was analyzed from a class of strains of Escherichia coli K12 in which the lac operon has been fused by deletion to the trp operon. These are strains that have retained the ability to synthesize tryptophan. Two of these strains are shown to make a wild-type trpA product; these strains retain intact all structural genes of the ttrp operon. It is proposed that the lac operon in these strains is fused to a region of the trp operon between trpA, the last gene in the operon, and the region where trp messenger RNA synthesis terminates. The region where trp messenger RNA synthesis terminates thus is distinct from the trp structural genes.     

Mutants of Escherichia coli with an Altered Tryptophanyl-Transfer Ribonucleic Acid Synthetase

Fourteen mutant strains of Escherichia coli were examined, each of which requires tryptophan for growth but is unaltered in any of the genes of the tryptophan biosynthetic operon. The genetic lesions responsible for tryptophan auxotrophy in these strains map between str and malA. Extracts of these strains have little or no ability to charge transfer ribonucleic acid (tRNA) with tryptophan. We found that several of the mutants produce tryptophanyl-tRNA synthetases which are more heat-labile than the enzyme of the parental wild-type strain. Of these heat-labile synthetases, at least one is protected against thermal inactivation by tryptophan, magnesium, and adenosine triphosphate. Two other labile synthetases which are not noticeably protected against heat inactivation by substrate have decreased affinity for tryptophan. On low levels of supplied tryptophan, these mutants exhibit markedly decreased growth rates but do not contain derepressed levels of the tryptophan biosynthetic enzymes. This suggests that the charging of tryptophan-specific tRNA is not involved in repression, a conclusion which is further substantiated by our finding that 5-methyltryptophan, a compound which represses the tryptophan operon, is not attached to tRNA by the tryptophanyl-tRNA synthetase of E. coli.     

Molecular basis of the indole-negative reaction in Shigella strains: extensive damages to the tna operon by insertion sequences

The molecular basis of the loss of tryptophan utilization (indole-negative phenotype) of Shigella strains, in effect clones of Escherichia coli, was investigated. Analysis of the tna operon of 23 Shigella strains representing each of the indole-negative serotypes revealed that insertion sequence-mediated insertion and/or deletions damaged the tna operon, leading to inability to convert tryptophan to indole. These events differ for cluster 1, cluster 3, and the outlier Shigella strains, confirming our previous observation of independent origins of these lineages from within E. coli. Parallel loss of the trait and prevalence of indole-negative strains suggest that the trait is deleterious in Shigella strains and advantages those without it.     

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

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.     

Temperature-sensitive repression of the tryptophan operon in Escherichia coli

Mutants of Escherichia coli exhibiting temperature-sensitive repression of the tryptophan operon have been isolated among the revertants of a tryptophan auxotroph, trpS5, that produces an altered tryptophanyl transfer ribonucleic acid (tRNA) synthetase. Unlike the parental strain, these mutants grew in the absence of tryptophan at high but not at low temperature. When grown at 43.5 C with excess tryptophan (repression conditions), they produced 10 times more anthranilate synthetase than when grown at 36 C or lower temperatures. Similar, though less striking, temperature-sensitivity was observed with respect to the formation of tryptophan synthetase. Transduction mapping by phage P1 revealed that these mutants carry a mutation cotransducible with thr at 60 to 80%, in addition to trpS5, and that the former mutation is primarily responsible for the temperature-sensitive repression. These results suggest that the present mutants represent a novel type of mutation of the classical regulatory gene trpR, which probably determines the structure of a protein involved in repression of the tryptophan operon. In agreement with this conclusion, tRNA of several trpR mutants was found to be normal with respect to its tryptophan acceptability. It was also shown that the trpS5 allele, whether present in trpR or trpR(+) strains, produced appreciably higher amounts of anthranilate synthetase than the corresponding trpS(+) strains under repression conditions. This was particularly true at higher temperatures. These results provide further evidence for our previous conclusion that tryptophanyl-tRNA synthetase is somehow involved in repression of this operon.     

Dynamic model of Escherichia coli tryptophan operon shows an optimal structural design.

A mathematical model has been developed to study the effect of external tryptophan on the trp operon. The model accounts for the effect of feedback repression by tryptophan through the Hill equation. We demonstrate that the trp operon maintains an intracellular steady-state concentration in a fivefold range irrespective of extracellular conditions. Dynamic behavior of the trp operon corresponding to varying levels of extracellular tryptophan illustrates the adaptive nature of regulation. Depending on the external tryptophan level in the medium, the transient response ranges from a rapid and underdamped to a sluggish and highly overdamped response. To test model fidelity, simulation results are compared with experimental data available in the literature. We further demonstrate the significance of the biological structure of the operon on the overall performance. Our analysis suggests that the tryptophan operon has evolved to a truly optimal design.     

Metabolic design based on a coupled gene expression-metabolic network model of tryptophan production in Escherichia coli

The presumably high potential of a holistic design approach for complex biochemical reaction networks is exemplified here for the network of tryptophan biosynthesis from glucose, a system whose components have been investigated thoroughly before. A dynamic model that combines the behavior of the trp operon gene expression with the metabolic network of central carbon metabolism and tryptophan biosynthesis is investigated. This model is analyzed in terms of metabolic fluxes, metabolic control, and nonlinear optimization. We compare two models for a wild-type strain and another model for a tryptophan producer. An integrated optimization of the whole network leads to a significant increase in tryptophan production rate for all systems under study. This enhancement is well above the increase that can be achieved by an optimization of subsystems. A constant ratio of control coefficients on tryptophan synthesis rate has been identified for the models regarding or disregarding trp operon expression. Although we found some examples where flux control coefficients even contradict the trends of enzyme activity changes in an optimized profile, flux control can be used as an indication for enzymes that have to be taken into account in optimization.     

Cyclic AMP synthesis in Escherichia coli strains bearing known deletions in the pts phosphotransferase operon

A series of isogenic strains harboring known deletions in the pts operon of Escherichia coli have been constructed by reverse genetics. Strains bearing deletions for the whole pts operon failed to grow on maltose or on carbon sources of the same class. In these strains the total cAMP synthesis was significantly lower than in a strain deleted only for the crr gene. This indicated that enzyme I or phosphorylated histidine-containing phosphotransferase protein in addition to its role in phosphorylating enzyme IIIGlc, is involved in adenylate cyclase (AC) activation or cAMP excretion. It was further shown that deletions in the pts operon do not affect synthesis of AC.     

New approach to tryptophan production by Escherichia coli: genetic manipulation of composite plasmids in vitro.

For the purpose of studying the production of L-tryptophan by Escherichia coli, the deletion mutants of the trp operon (trpAE1) were transformed with mutant plasmids carrying the trp operon whose anthranilate synthase and phosphoribosyl anthranilate transferase (anthranilate aggregate), respectively, had been desensitized to tryptophan inhibition. In addition to release of the anthranilate aggregate from the feedback inhibition required for plasmids such as pSC101 trp.I15, the properties of trp repression (trpR) and tryptophanase deficiency (tnaA) were both indispensable for host strains such as strain Tna (trpAE1 trpR tnaA). The gene dosage effects on tryptophan synthase activities and on production of tryptophan were assessed. A moderate plasmid copy number, approximately five per chromosome, was optimal for tryptophan production. Similarly, an appropriate release of the anthranilate aggregate from feedback inhibition was also a necessary step to ward off the metabolic anomaly. If the mutant plasmid pSC101 trp-I15 was further mutagenized (pSC101 trp.I15.14) and then transferred to Tna cells, an effective enhancement of tryptophan production was achieved. Although further improvement of the host-plasmid system is needed before commercial production of tryptophan can be realized by this means, a promising step toward this goal has been established.