If a particular operon encodes enzymes for making an essential amino acid and is regulated like the trp operon, then a. the amino acid inactivates the repressor. b. the repressor is active in the absence of the amino acid. c. the amino acid acts as a corepressor. d. the amino acid turns on transcription of the operon.

Verified step by step guidance

1. Understand the function of the trp operon: The trp operon is a group of genes that are used to synthesize tryptophan, an essential amino acid, in bacteria. It is regulated by feedback inhibition.
2. Identify the role of the repressor and corepressor in the trp operon: In the trp operon, the repressor protein is inactive by itself and requires the presence of tryptophan (the corepressor) to bind to the operator site and inhibit gene transcription.
3. Analyze the options in relation to the trp operon model: Option (a) suggests the amino acid inactivates the repressor, which is incorrect as the amino acid (tryptophan) activates the repressor by binding to it. Option (b) is incorrect as the repressor is inactive without the amino acid. Option (d) is incorrect because the amino acid does not turn on transcription; it actually helps turn it off.
4. Identify the correct answer based on the trp operon regulation: Since the amino acid (tryptophan) acts as a corepressor by binding to the repressor and enabling it to bind to the operator to stop transcription, the correct answer is (c) the amino acid acts as a corepressor.
5. Confirm the answer: The correct mechanism of regulation for an operon encoding enzymes for making an essential amino acid, regulated like the trp operon, involves the amino acid acting as a corepressor. Therefore, the correct answer is (c).

Key Concepts

Operon Structure

An operon is a cluster of genes under the control of a single promoter, allowing for coordinated regulation of gene expression. In prokaryotes, operons enable the efficient use of resources by regulating the transcription of multiple genes that encode proteins with related functions, such as enzymes involved in a metabolic pathway.

Repressor Proteins

Repressor proteins are regulatory proteins that bind to specific DNA sequences, inhibiting the transcription of target genes. In the context of operons, the presence or absence of certain metabolites, like amino acids, can influence the activity of these repressors, determining whether the operon is active or inactive.

Corepressor Function

A corepressor is a small molecule that binds to a repressor protein, enhancing its ability to bind to the operator region of an operon. In the case of the trp operon, the presence of tryptophan acts as a corepressor, allowing the repressor to effectively block transcription when the amino acid is abundant, thus regulating the synthesis of enzymes needed for its production.