The following hypothetical genotypes have genes A, B, and C corresponding to lacI, lacO, and lacZ, but not necessarily in that order. Data in the table indicate whether -galactosidase is produced in the presence and absence of the inducer for each genotype. Use these data to identify the correspondence between A, B, and C and the lacI, lacO, and lacZ genes. Carefully explain your reasoning for identifying each gene.

Genotype                  β-Galactosidase Production
                             Inducer Present       Inducer Absent
1. A⁻B⁺C⁺⁻                          +                              +
2. A⁺B⁺C⁻                           +                              +
3.A⁻B⁺C⁺/A⁺B⁺C⁺                +                              +
4. A⁺B⁺C/A⁺B⁺C⁺⁻                +                              –

Key Concepts

Lac Operon

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of three structural genes: lacZ, lacY, and lacA, which encode proteins necessary for lactose utilization. The operon is regulated by the lacI gene, which produces a repressor that inhibits transcription in the absence of lactose. Understanding the lac operon is crucial for analyzing how the presence or absence of an inducer affects β-galactosidase production.

Gene Function and Interaction

Each gene in the lac operon has a specific function: lacZ encodes β-galactosidase, which breaks down lactose; lacY encodes permease, facilitating lactose entry into the cell; and lacA encodes transacetylase, which is less understood. The interaction between these genes and their products determines the operon's overall function. Analyzing the genotypes provided helps in deducing which genes correspond to A, B, and C based on their effects on β-galactosidase production.

Inducer Role

An inducer, such as allolactose, binds to the lac repressor, causing it to release from the operator region of the lac operon. This allows RNA polymerase to transcribe the structural genes, leading to the production of β-galactosidase. The presence or absence of the inducer in the experimental data is critical for determining the functionality of the genes represented by A, B, and C, as it directly influences whether β-galactosidase is produced.