In a theoretical operon, genes A, B, C, and D represent the repressor gene, the promoter sequence, the operator gene, and the structural gene, but not necessarily in the order named. This operon is concerned with the metabolism of a theoretical molecule (tm). From the data provided in the accompanying table, first decide whether the operon is inducible or repressible. Then assign A, B, C, and D to the four parts of the operon. Explain your rationale. (AE=active enzyme; IE=inactive enzyme; NE=no enzyme.) Genotype tm Present tm Absent A⁺B⁺C⁺D⁺ AE NE A⁻B⁺C⁺D⁺ AE AE A⁺B⁻C⁺D⁺ NE NE A⁺B⁺C⁻D⁺ IE NE A⁺B⁺C⁺D⁻ AE AE A⁻B⁺C⁺D⁺/F'A⁺B⁺C⁺D⁺ AE AE A⁺B⁻C⁺D⁺/F'A⁺B⁺C⁺D⁺ AE NE A⁺B⁺C⁻D⁺/F'A⁺B⁺C⁺D⁺ AE+IE NE A⁺B⁺C⁺D⁻/F'A⁺B⁺C⁺D⁺ AE NE

Verified step by step guidance

1. Identify whether the operon is inducible or repressible by analyzing the enzyme activity in the presence and absence of tm. If the enzyme is active only in the presence of tm, it is inducible. If it is inactive in the presence of tm, it is repressible.
2. Examine the genotype A⁺B⁺C⁺D⁺: Active enzyme (AE) is produced when tm is present, and no enzyme (NE) is produced when tm is absent, suggesting an inducible operon.
3. Determine the role of each gene by analyzing the effects of mutations: A⁻B⁺C⁺D⁺ results in AE regardless of tm presence, indicating A is the repressor gene.
4. Analyze the genotype A⁺B⁻C⁺D⁺: NE is produced regardless of tm presence, suggesting B is the promoter sequence, as its mutation prevents transcription.
5. Consider the genotype A⁺B⁺C⁻D⁺: Inactive enzyme (IE) is produced when tm is present, indicating C is the operator gene, as its mutation affects enzyme activity. D is the structural gene, as its mutation (A⁺B⁺C⁺D⁻) results in AE regardless of tm presence.

Key Concepts

Operon Structure

An operon is a cluster of genes under the control of a single promoter, which includes regulatory elements such as the repressor, operator, and structural genes. The repressor gene produces a protein that can bind to the operator, blocking transcription of the structural genes. Understanding the arrangement and function of these components is crucial for determining how the operon responds to the presence or absence of specific molecules.

Inducible vs. Repressible Operons

Inducible operons are typically off and can be turned on in response to an inducer, while repressible operons are usually on and can be turned off by a corepressor. The distinction is important for understanding metabolic pathways; for example, the lac operon is inducible, while the trp operon is repressible. Analyzing the data provided helps in identifying the nature of the operon in question.

Enzyme Activity and Genotype

The activity of enzymes in response to different genotypes provides insight into the operon's functionality. The presence of active (AE), inactive (IE), or no enzyme (NE) under varying conditions indicates how the operon is regulated. By correlating the genotype with enzyme activity in the presence or absence of the theoretical molecule (tm), one can deduce the roles of genes A, B, C, and D in the operon.