Question

A repressible operon system, like the trp operon, contains three genes, G, Z, and W. Operon genes are synthesized when the end product of the operon synthesis pathway is absent, but there is no synthesis when the end product is present. One of these genes is an operator, one is a regulatory protein, and the other is a structural enzyme involved in synthesis of the end product. In the table below, '+' indicates that the enzyme is synthesized by the operon and '−' means that no enzyme synthesis occurs. Use this information to determine which gene corresponds to each operon function.
Table displaying genotypes and enzyme synthesis results based on end product presence or absence.

Table displaying genotypes and enzyme synthesis results based on end product presence or absence.

Accepted Answer

Step 1: Understand the concept of a repressible operon system. A repressible operon, such as the trp operon, is regulated by the presence or absence of the end product of its synthesis pathway. When the end product is absent, the operon is active, and the genes are transcribed to synthesize the enzyme. When the end product is present, it binds to the regulatory protein, which then inhibits transcription by interacting with the operator. Step 2: Identify the roles of the three genes in the operon system. The operon typically consists of: (1) a structural gene that codes for the enzyme involved in the synthesis of the end product, (2) a regulatory gene that codes for the regulatory protein, and (3) an operator sequence that interacts with the regulatory protein to control transcription. Step 3: Analyze the table provided in the problem. The table indicates whether the enzyme is synthesized ('+') or not synthesized ('−') under different conditions. Use this information to deduce the function of each gene based on its behavior in the presence or absence of the end product. Step 4: Match the gene functions to the operon components. The structural gene will be responsible for enzyme synthesis, the regulatory gene will produce the protein that interacts with the operator, and the operator will be the sequence that controls transcription based on the regulatory protein's activity. Step 5: Verify your reasoning by ensuring that the behavior of each gene aligns with the repressible operon model. For example, the regulatory protein should inhibit transcription when the end product is present, and the operator should respond to the regulatory protein to control enzyme synthesis.

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Key Concepts

Repressible Operon

Operon Structure

Gene Functions in Operons