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Pathway Description
Trp Operon
Escherichia coli
Category:
Metabolite Pathway
Sub-Category:
Signaling
Created: 2015-07-02
Last Updated: 2019-08-16
The trp operon in E. coli contains five genes that produce proteins that are used in the production of the amino acid tryptophan when needed by the cell. When tryptophan levels in the cell are high, tryptophan binds to the trp operon repressor protein, which activates it. The activated repressor then binds to the operator, preventing RNA polymerase from binding and transcribing the operon. However, when tryptophan concentrations in the cell are low, it doesn't bind to the repressor, preventing it from binding to the operator, and allowing transcription until the terminator after the trpA gene is reached.
The trp operon is also regulated by the amount of useable trp tRNA present. Upon start of transcription, the leader peptide, encoded by the trpL gene, will begin to be transcribed. Because this peptide contains two trp residues next to each other, and trp is a relatively uncommon amino acid, if there is a low concentration of trp tRNA in the cell, it can cause the leader peptide to stall during transcription. This allows for the section of mRNA immediately after the stalled ribosome to form the anti-termination hairpin. This hairpin prevents the formation of the terminal hairpin that contains a termination sequence that would stop transcription after the leader peptide. Because the anti-termination hairpin is allowed to form, transcription of the rest of the operon can continue. However, when the cell contains a high concentration of trp tRNA, the transcription does not stall, which allows for the formation of the transcription terminator to form before the rest of the genes in the operon, preveinting their transcription.
The trpE and trpD genes encode for anthranilate synthase components 1 and 2 respectively. These combine to create anthranilate synthase, which produces anthranilate and pyruvate from chorismate.
The trpC gene encodes the tryptophan biosynthesis protein that takes the anthranilate from the previous protein and converts it in two steps to indole-3-glycerol.
Finally, the trpB and trpA genes encode for tryptophan beta and alpha subunits respectively. Two of each subunit come together to form tryptophan synthase. This protein then takes the previous compound, as well as a molecule of L-serine, and catalzes their conversion into tryptophan, as well as water and D-glyceraldehyde-3-phosphate.
References
Trp Operon References
Robert F. Weaver, Molecular Biology, 4th edition McGrawHill
Merino E, Jensen RA, Yanofsky C: Evolution of bacterial trp operons and their regulation. Curr Opin Microbiol. 2008 Apr;11(2):78-86. doi: 10.1016/j.mib.2008.02.005.
Pubmed: 18374625
Gollnick P, Yanofsky C: tRNA(Trp) translation of leader peptide codon 12 and other factors that regulate expression of the tryptophanase operon. J Bacteriol. 1990 Jun;172(6):3100-7. doi: 10.1128/jb.172.6.3100-3107.1990.
Pubmed: 2345136
Yanofsky C, Platt T, Crawford IP, Nichols BP, Christie GE, Horowitz H, VanCleemput M, Wu AM: The complete nucleotide sequence of the tryptophan operon of Escherichia coli. Nucleic Acids Res. 1981 Dec 21;9(24):6647-68. doi: 10.1093/nar/9.24.6647.
Pubmed: 7038627
Nichols BP, van Cleemput M, Yanofsky C: Nucleotide sequence of Escherichia coli trpE. Anthranilate synthetase component I contains no tryptophan residues. J Mol Biol. 1981 Feb 15;146(1):45-54. doi: 10.1016/0022-2836(81)90365-x.
Pubmed: 7021857
Aiba H, Baba T, Hayashi K, Inada T, Isono K, Itoh T, Kasai H, Kashimoto K, Kimura S, Kitakawa M, Kitagawa M, Makino K, Miki T, Mizobuchi K, Mori H, Mori T, Motomura K, Nakade S, Nakamura Y, Nashimoto H, Nishio Y, Oshima T, Saito N, Sampei G, Horiuchi T, et al.: A 570-kb DNA sequence of the Escherichia coli K-12 genome corresponding to the 28.0-40.1 min region on the linkage map. DNA Res. 1996 Dec 31;3(6):363-77. doi: 10.1093/dnares/3.6.363.
Pubmed: 9097039
Horowitz H, Christie GE, Platt T: Nucleotide sequence of the trpD gene, encoding anthranilate synthetase component II of Escherichia coli. J Mol Biol. 1982 Apr 5;156(2):245-56. doi: 10.1016/0022-2836(82)90326-6.
Pubmed: 6283099
Christie GE, Platt T: Gene structure in the tryptophan operon of Escherichia coli. Nucleotide sequence of trpC and the flanking intercistronic regions. J Mol Biol. 1980 Oct 5;142(4):519-30. doi: 10.1016/0022-2836(80)90261-2.
Pubmed: 7007653
Horowitz H, Van Arsdell J, Platt T: Nucleotide sequence of the trpD and trpC genes of Salmonella typhimurium. J Mol Biol. 1983 Oct 5;169(4):775-97. doi: 10.1016/s0022-2836(83)80136-3.
Pubmed: 6355484
Zhao GP, Somerville RL: Genetic and biochemical characterization of the trpB8 mutation of Escherichia coli tryptophan synthase. An amino acid switch at the sharp turn of the trypsin-sensitive "hinge" region diminishes substrate binding and alters solubility. J Biol Chem. 1992 Jan 5;267(1):526-41.
Pubmed: 1309752
Milkman R, Bridges MM: Molecular evolution of the Escherichia coli chromosome. IV. Sequence comparisons. Genetics. 1993 Mar;133(3):455-68.
Pubmed: 8095913
Guest JR, Drapeau GR, Carlton BC, Yanofsky C: The amino acid sequence of the A protein (alpha subunit) of the tryptophan synthetase of Escherichia coli. J Biol Chem. 1967 Nov 25;242(22):5442-6.
Pubmed: 4863752
Nichols BP, Yanofsky C: Nucleotide sequences of trpA of Salmonella typhimurium and Escherichia coli: an evolutionary comparison. Proc Natl Acad Sci U S A. 1979 Oct;76(10):5244-8. doi: 10.1073/pnas.76.10.5244.
Pubmed: 388433
Squires C, Lee F, Bertrand K, Squires CL, Bronson MJ, Yanofsky C: Nucleotide sequence of the 5' end of tryptophan messenger RNA of Escherichia coli. J Mol Biol. 1976 May 15;103(2):351-81. doi: 10.1016/0022-2836(76)90317-x.
Pubmed: 781271
Oxender DL, Zurawski G, Yanofsky C: Attenuation in the Escherichia coli tryptophan operon: role of RNA secondary structure involving the tryptophan codon region. Proc Natl Acad Sci U S A. 1979 Nov;76(11):5524-8. doi: 10.1073/pnas.76.11.5524.
Pubmed: 118451
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