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What Is the Tryptophan Operon?

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  • Written By: Andrew Kirmayer
  • Edited By: Angela B.
  • Last Modified Date: 01 September 2014
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A tryptophan operon, also known as a trp operon, is a region of a DNA molecule in which genes that encode for enzymes for tryptophan are expressed when amino acid is low. Tryptophan is one of 20 amino acids used inside of living cells. Its production has been well studied within Escherichia coli (E. coli) bacteria.

In the E. coli cell, five genes code for tryptophan. These are linked together, side by side, in a structure called an operon. The tryptophan operon also consists of and is controlled by an operator site. DNA transcription is controlled by proteins. Repressor proteins can attach to the operator site, blocking the genes from being transcribed and tryptophan from being produced.

These repressing proteins are inactive when the amino acid is not present, so they do not attach to the operator site to block production. The operator site is linked to a promoter site on the tryptophan operon, where a molecule called RNA polymerase attaches and can proceed along the gene group to transcribe the genes. This generates messenger RNA that codes for tryptophan. This transcription process is the basis for production of this amino acid.

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When this amino acid is present, it binds to the repressor protein, which is activated and binds to the operator site to block any transcription along the tryptophan operon. To simplify this description, the amino acid molecules actually change the shape of the repressor so that it is able to fit inside the helix shape of the DNA. This is how the molecular procession is blocked. When tryptophan is not present, the repressor is shaped differently and cannot fit into the DNA. The function of the repressor in turning off gene transcription is referred to as negative control.

The trp operon is one of many biosynthetic operons in the genomes of every living thing. Tryptophan is known as a co-repressor, because it attaches to the repressing protein to stop transcription along the tryptophan operon. In the human body, tryptophan is one of 10 amino acids involved in protein synthesis. A precursor for serotonin, which regulates sleep patterns, mood, and appetite, it has an important role. Deficiencies in this amino acid can result in serotonin deficiencies, which then results in symptoms such as depression, anxiety, weight gain, and insomnia.

An excess of tryptophan is not considered toxic, but it causes tiredness. It is in most foods that have protein, especially turkey, bananas, soybeans, and tuna. Regulated by the activation and repression of the tryptophan operon, this amino acid has an essential function in all living cells, such as E. coli, as well as in the human body.

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