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

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.