How is the Lac Operon Regulated

Gene expression is the synthesis of a polypeptide chain of a functional protein based on the information encoded by a particular gene. The amount of the synthesis of a particular protein can be regulated by the regulation of the gene expression. The differential expression of genes can be achieved during the various steps of the protein synthesis. However, the regulation of the gene expression is different in eukaryotic and prokaryotic genes. Lac operon is a cluster of genes responsible for the lactose metabolism of E.coli. The regulation of the expression of lac operon is achieved in response to the lactose and glucose levels in the medium. The regulation of the lac operon is used as the foremost example of prokaryotic gene regulation in introductory molecular and cellular biology studies.

Key Areas Covered

1. What is Regulation of Gene Expression
      – Definition, Regulation of Gene Expression
2. What is the Lac Operon
     – Definition, Structure, Function of Gene Products
3. How is the Lac Operon Regulated
     – Lac repressor, CAP

Key Terms: Catabolite Activator Protein (CAP), E. coli, Gene Expression, Glucose, Lac Operon, Lac Repressor, Lactose Metabolism

What is Regulation of Gene Expression

The regulation of gene expression refers to a wide range of mechanisms used by the cell to either increase or decrease the production of a particular gene product (a protein or an RNA). It is achieved during various steps of the protein synthesis as described below.

1. Replication level – The mutations that occur during DNA replication may cause alterations of the gene expression.
2. Transcriptional level – The transcription of a particular gene can be controlled by repressors and activators.
3. Post-transcriptional level – Gene expression can be achieved during the post-transcriptional modifications such as RNA splicing.
4. Translational level – The translation of a mRNA molecule can be controlled by various processes such as RNA interference pathway.
5. Post-translational level – The synthesis of a protein can be regulated at the post-translational level by controlling the post-translational modifications.

However, the regulation of gene expression in prokaryotes is mainly achieved during the initiation of transcription. It involves the activators that positively-regulates the gene expression and repressers that negatively-regulate the gene expression. The regulation of the gene expression at different steps of the protein synthesis is shown in figure 1.

Figure 1: Regulation of Gene Expression

What is Lac Operon

The lac operon refers to a cluster of genes responsible for the lactose metabolism of E. coli. Hence, the lac operon is a functional unit of the E. coli genome. All the genes in the lac operon are controlled by a single promoter. Hence, all the genes in the operon are transcribed together. The gene products are the proteins responsible for transporting lactose into the cytosol of the cell and digestion of lactose into glucose. Glucose is used in the cellular respiration to produce energy in the form of ATP. The lac operon may be present in many other enteric bacteria as well. The structure of the lac operon is shown in figure 2.

Figure 2: Lac Operon

The lac operon is made up of three genes controlled by a single promoter. These genes are lacZ, lacY, and lacA. These genes are encoded for the three enzymes involved in the lactose metabolism known as beta-galactosidase, beta-galactoside permease, and beta-galactoside transacetylase respectively. Beta-galactosidase is involved in the breakdown of lactose into glucose and galactose. Beta-galactoside permease is embedded in the cell membrane, enabling the transport of lactose into the cytosol. Beta-galactoside transacetylase is involved in the transfer of an acetyl group from acetyl Co-A to beta-galactoside. The transcription of the lac operon produces a polycistronic mRNA molecule that produces all the three gene products from a single mRNA molecule. Generally, the lacZ and lacY gene products are sufficient for lactose catabolism.

In addition to those three genes, lac operon is composed of a number of regulatory regions to which various proteins can bind to control the transcription. The key regulatory sequences in the lac operon are the promoter, operator, and the catabolite activator protein (CAP) binding site. The promoter serves as the binding site for the RNA polymerase, the enzyme responsible for the transcription of the genes. The operator serves as a negative regulatory site to which the lac repressor binds. The CAP binding site serves as the positive regulatory site to which the CAP binds.

How is the Lac Operon Regulated

The regulation of the gene expression in prokaryotic genes occurs by means of inducible operons in which different types of proteins bind, either activating or repressing the transcription of the operon based on the requirements of the cell. Lac operon is an inducible operon. It allows the usage of lactose, a disaccharide, in the energy production by converting it into glucose that can be readily used in the cellular respiration, when the glucose is not available for the cell. The lac operon is regulated in “turn off” and “turn on” states based on the presence of glucose in the cell. The lac repressor is responsible for the ‘turn off’ mode of the lac operon while CAP is responsible for the ‘turn on’ mode of the lac operon.

Lac Repressor

The lac repressor refers to a lactose sensor, which blocks the transcription of the lac operon in the presence of glucose. The usage of glucose in the cellular respiration requires fewer steps in the production of energy when compared to lactose. Hence, when glucose is available in the cell, it is readily broken down in the cellular pathways to produce energy. In addition, when glucose is used in the respiration, the usage of the lactose for the former purpose should be avoided in order to achieve the maximum efficiency of the cellular respiration. In this situation, the blockage of the transcription of the lac operon is achieved by the binding of lac repressor to the operator region of the lac operon. Generally, the operator region overlaps with the promoter region. Hence, when the lac repressor binds to the operator region, RNA polymerase is incapable of binding to the promoter region as the complete promoter region is not available. When glucose is readily available in the cell and lactose is not available, the lac repressor tightly binds to the operator region, inhibiting the transcription of the lac operon. The regulation of the lac operon is shown in figure 3.

Figure 3: Regulation of the Lac Operon

Catabolite Activator Protein (CAP)

The CAP protein refers to a glucose repressor that activates the transcription of the lac operon. When the cell runs out of glucose and lactose is readily available inside the cytosol, the lac repressor losses its ability to bind with the DNA. Hence, it floats off from the operator region, making the promoter region available for the binding to RNA polymerase. When lactose is available, some of the molecules are converted into allolactose, a small isomer of lactose. The binding of the allolactose to the lac repressor causes the loosening of it from the operator region. Therefore, allolactose serves as an inducer, triggering the expression of the lac operon. Further, the lac operon is considered as an inducible operon as well.

However, RNA polymerase alone is unable to bind perfectly to the promoter region. Hence, CAP aids in the tight binding of the RNA polymerase to the promoter. It binds to the CAP binding site upstream to the promoter. The binding of the CAP to the DNA is regulated by a small molecule known as the cyclic AMP (cAMP). The cAMP serves as the hunger signal made by E. coli in the absence of glucose. The binding of the cAMP to CAP changes the conformation of CAP, enabling the binding of CAP to the CAP binding site of the lac operon. However, cAMP is present in the cell when glucose levels are very low inside the cell. Hence, the activation of the lac operon can only be achieved when glucose is not available for the cell. In conclusion, the activation of the lac operon can be achieved when glucose is not available and lactose is available inside the cell. When both glucose as well as lactose are absent in the cell, the lac repressor remains binding to the lac operon, preventing the transcription of the operon.

Conclusion

The lac operon is an inducible operon where the proteins required by the lactose metabolism are present in clusters of genes. Hence, the transcription of the lac operon produces a polycistronic mRNA molecule capable of synthesizing multiple gene products. The lac operon is expressed only in the absence of glucose and the presence of lactose inside the cell for cellular respiration. The lac repressor binds to the operator region of the lac operon when glucose is readily available, and lactose is unavailable. The CAP binds to the operator of the lac operon, aiding the transcription when glucose is unavailable, and lactose is readily available. Hence, the cell becomes capable of utilizing lactose in the cellular respiration to produce energy.

Image Courtesy:

1. “Gene expression control” By ArneLH – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Lac operon1” (Public Domain) via Commons Wikimedia
3. “Lac operon” (CC BY 2.0) via Commons Wikimedia

Reference:

1.“Prokaryotic Gene Regulation.” Lumen / Boundless Biology, Available here.
2.“The lac operon.” Khan Academy, Available here.
3.“Lac Operon : Regulation of Gene Expression in Prokaryotes.” Biology, Byjus Classes, 21 Nov. 2017, Available here.