Tryptophan repressor

Tryptophan Repressor

The tryptophan repressor is a protein found in Escherichia coli that regulates the expression of the tryptophan operon. It is a classic example of a repressor protein that controls gene expression in response to environmental conditions.

Structure

The tryptophan repressor is a homodimer, meaning it consists of two identical subunits. Each subunit is composed of 107 amino acids. The protein binds to DNA at the operator region of the tryptophan operon, preventing transcription of the downstream genes. The repressor has a helix-turn-helix motif, which is a common structural motif in DNA-binding proteins.

Function

The primary function of the tryptophan repressor is to regulate the tryptophan operon, which contains genes necessary for the synthesis of the amino acid tryptophan. When tryptophan levels are high, tryptophan molecules bind to the repressor, causing a conformational change that increases its affinity for the operator region of the operon. This binding prevents RNA polymerase from transcribing the operon, thus inhibiting the production of tryptophan.

Mechanism of Action

The tryptophan repressor operates through a negative feedback loop. In the absence of tryptophan, the repressor is inactive and does not bind to the operator, allowing the operon to be transcribed. As tryptophan accumulates, it binds to the repressor, activating it. The activated repressor then binds to the operator, blocking transcription. This mechanism ensures that the cell does not waste resources producing tryptophan when it is already abundant.

Biological Significance

The regulation of the tryptophan operon by the tryptophan repressor is an example of feedback inhibition, a common regulatory mechanism in prokaryotes. This system allows E. coli to efficiently manage its resources and adapt to changes in nutrient availability.

Related Pages

• Operon
• Gene regulation
• Helix-turn-helix
• Feedback inhibition

References

• Beckwith, J. (1987). "The operon: An historical account." In Escherichia coli and Salmonella typhimurium: Cellular and Molecular Biology. American Society for Microbiology.
• Yanofsky, C. (1981). "Attenuation in the control of expression of bacterial operons." Nature 289, 751-758.

The structure of the tryptophan repressor bound to DNA.