Question

Why does the lac operon shut down sometime after the addition of lactose in the medium where E. coli was growing? Why is the low level expression of the lac operon always required?

Answer 1

From their study in bacterial genetics, Jacob and Monod in 1961, proposed that an operator is a functional unit of genetic material. An operon consists of structural genes, whose transcription is regulated by the action of the regulator gene, promoter gene, operator gene, and terminator gene.

Complete answer:
1. Structural gene: Directly involved in the synthesis of cellular proteins. Structural genes produce RNAs by the process of transcription and determine the sequence of amino acids in the synthesized proteins.
2. Operator gene: Its position is adjacent to the first structural gene. It determines the repression of the structural gene by the repressor protein- a product of the regulator gene.
Promoter gene: It is located just before the operator. According to Pribnow, the promoter region has three essential elements; a recognition sequence, a RNA polymerase binding sequence, and an mRNA initiation site.
3. Regulator gene: These genes direct the synthesis of an active repressor or an inactive repressor protein. Repressor protein has two active sites, one for operator recognition and the other for inducer. In the absence of an inducer protein, the active repressor protein binds to the operator gene and thus inhibits the binding of RNA polymerase.
Lac operon shuts down sometime after the addition of lactose in the medium because, after the addition of lactose, beta-galactosidase performs the complete breakdown of lactose into glucose and galactose. Therefore, there is no more lactose present in the medium and lactose will no longer bind to the repressor protein and as a result, the lac operon shuts down. A low-level expression of the lac operon is always required in the cell at all times, otherwise, lactose can’t enter the cell.

Note:
An inducer or an effector is a small sugar, amino acid, or nucleotide molecule that can be linked to a regulator or repressor protein and changes its ability to interact with an operator or a promoter gene. A regulator protein binds to an operator and inhibits operon expression, as soon as the effector molecule appears in the intracellular environment, it will bind to the regulator protein and pull it away from the operator.