Question

Mention the differences between the mechanisms of the regulation of gene activity in lac operon and trp operon in E. coli

Answer 1

An operon is a cluster of functionally-related genes that are controlled by a shared operator. Operons consist of multiple genes grouped together with a promoter and an operator. For example, the bacterium Escherichia coli contains a number of genes clustered into operons and regulons: the Lac operon which is involved in lactose degradation, the Trp operon which is involved in tryptophan biosynthesis, and the His operon which is involved in histidine biosynthesis.

The Lac operon is an inducible operon; in the absence of lactose the operator is blocked by a repressor protein. The operon is made up of a promoter with operator, and three genes (lacZ, lacY, and lacA) which encode β-galactosidase, permease, and transacetylase. The three genes are involved in the breakdown of lactose into its metabolites: β-galactosidase breaks lactose down into glucose and galactose, while the other two proteins aid in the metabolic process. The expression of the Lac operon is controlled by the regulatory gene lacI, located immediately adjacent to the promoter region. LacI encodes an allosteric repressor protein that keep the Lac operon “off”.

In order for the Lac operon to be turned on, an inducer molecule must inactivate the repressor protein. The inducer molecule in this system is allolactose, an isomer of lactose. When lactose and its isomer are present in the cell, allolactose will bind to allosteric sites on the repressor protein, changing its conformation and rendering it inactive. As the repressor protein detaches from the operator, RNA polymerase can bind to the promoter, transcription can occur, and the three lactose degradation genes can be synthesized.

The Trp operon is responsible for synthesis of the amino acid trytophan when it is not available in the environment. The Trp operon is made up of a promoter with an operator, and five genes that encode enzymes for tryptophan synthesis. The Trp operon is regulated by the regulatory gene trpR, a gene that is located at a distance from the Trp operon.

The Trp operon is an example of a repressible operon; it is on unless turned off by a repressor protein. The repressor protein is synthesized by trpR. While the repressor protein is always present in the cell, it is synthesized in an inactive form. When a corepressor is present, in this case tryptophan, it binds to the repressor protein in an allosteric site. This changes the conformation of the protein such that it can bind to the operator and block transcription by preventing the binding of RNA polymerase to the promoter. In this way the cell saves energy by not producing tryptophan when it is already present.