Tryptophan repressor

Last updated
Trp repressor protein
Ribbon diagram of the trpR protein
Pfam PF01371
Pfam clan CL0123
InterPro IPR000831
Trp operon repressor
Organism Escherichia coli
Entrez 948917
RefSeq (Prot) NP_418810
UniProt P0A881
Other data
Chromosome genome: 4.63 - 4.63 Mb

Tryptophan repressor (or trp repressor) is a transcription factor involved in controlling amino acid metabolism. It has been best studied in Escherichia coli , where it is a dimeric protein that regulates transcription of the 5 genes in the tryptophan operon. [1] When the amino acid tryptophan is plentiful in the cell, it binds to the protein, which causes a conformational change in the protein. [2] The repressor complex then binds to its operator sequence in the genes it regulates, shutting off the genes. [3] [4]


One of the genes regulated by trp repressor, trpR, codes for the tryptophan repressor protein itself. This is a form of feedback regulation. However, these genes are located on different operons.

The (tryptophan) repressor is a 25 kD protein homodimer which regulates transcription of the tryptophan biosynthetic pathway in bacteria. There are 5 operons which are regulated by trpR: the trpEDCBA, trpR, AroH, AroL, and mtr operons.


When the amino acid tryptophan is in plentiful supply in the cell, trpR binds 2 molecules of tryptophan, which alters its structure and dynamics so that it becomes able to bind to operator DNA. When this occurs, transcription of the DNA is prevented, suppressing the products of the gene - proteins which make more tryptophan. When the cellular levels of tryptophan decline, the tryptophan molecules on the repressor fall off, allowing the repressor to return to its inactive form.

trpR also controls the regulation of its own production, through regulation of the trpR gene. [5]

The structure of the ligand-bound holorepressor, and the ligand-free forms have been determined by both X-ray crystallography and NMR. [6] [7] [8] [9] [10]

The trp operon consists of a regulatory gene, a promoter, an operator, and a terminator. The trp operon is active only when cellular tryptophan is scarce. If there isn't enough tryptophan, the repressor protein breaks off from the operator (where the repressor is normally bound) and RNA polymerase can complete its reading of the strand of DNA. If the RNA polymerase reaches the terminator (at the end of the DNA strand), the enzymes for tryptophan biosynthesis are expressed.

See also

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In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product. The result of this is that the genes contained in the operon are either expressed together or not at all. Several genes must be co-transcribed to define an operon.

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François Jacob was a French biologist who, together with Jacques Monod, originated the idea that control of enzyme levels in all cells occurs through regulation of transcription. He shared the 1965 Nobel Prize in Medicine with Jacques Monod and André Lwoff.

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The lac repressor is a DNA-binding protein that inhibits the expression of genes coding for proteins involved in the metabolism of lactose in bacteria. These genes are repressed when lactose is not available to the cell, ensuring that the bacterium only invests energy in the production of machinery necessary for uptake and utilization of lactose when lactose is present. When lactose becomes available, it is converted into allolactose, which inhibits the lac repressor's DNA binding ability, thereby increasing gene expression.

<i>lac</i> operon Set genes encoding proteins and enzymes for lactose metabolism

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<i>trp</i> operon Operon that codes for the components for production of tryptophan

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