Transcriptional regulation mechanism of the Lactose operon when Lactose and Glucose coexist

1. Structural genes of the lactose operon.
2. Transcription regulatory gene and the repressor protein.
3. Transcription of the lac-operon in the presence of glucose and lactose.
4. Transcription of the lac-operon in the absence of glucose but only lactose.
5. Control of transcription frequency by the CAP-cAMP complex.

E. coli lactose operon contains three structural genes: lac-Z, lac-Y, and lac-A. These genes encode β-galactosidase, permease, and transacetylase, respectively.

β-galactosidase breaks down lactose, a disaccharide, into the monosaccharides glucose and galactose. Permease is an enzyme responsible for transporting lactose into the cell. The role of transacetylase in lactose metabolism is not well understood.

Upstream of the lactose operon is a regulatory gene called lacI. The “I” is believed to stand for “inducible”. The product of lacI is a repressor protein that inhibits transcription.

The transcript of the lactose operon is a polycistronic mRNA that includes the three protein-coding genes. Translation of each gene in this mRNA begins at its respective AUG codon and ends at its stop codon. The lacI gene is continuously transcribed and translated, ensuring a constant presence of the repressor protein in the cell.

First, I will explain the transcription of the lactose operon when lactose is not present in the medium. Since there is no lactose to metabolize, it is reasonable not to produce the enzymes necessary for this process.

Regardless of the presence or absence of lactose, the repressor protein is constantly supplied in the cytoplasm. This repressor protein has an affinity for the DNA sequence in the operator region, indicated as lacO in the regulatory region, and can strongly adhere to this sequence.

When the repressor protein attaches to the operator region, even if RNA polymerase attaches to the promoter region for transcription of the structural genes, it cannot proceed with the transcription of the downstream structural genes.

As a result, when lactose is not present in the medium, there is a rational mechanism in place where the transcription of the three structural genes necessary for lactose metabolism does not occur.

Let’s add a bit more explanation about repressor proteins. The product of the lacI gene is a repressor protein, and this repressor protein forms a tetramer.

The repressor protein in its tetrameric form strongly adheres to the DNA sequence in the operator region. It binds tightly and bends the DNA, forming a loop.

When DNA takes on this structure, even if RNA polymerase attaches to the upstream promoter sequence, it cannot transcribe the downstream region. In this way, the repressor, which is the product of lacI, inhibits the transcription of structural genes.

Next, I will explain the transcription of the lactose operon in a situation where lactose (a disaccharide) is present in the medium, and glucose (a monosaccharide) is also present at high concentrations.

When both the monosaccharide glucose and the disaccharide lactose are present in the medium, it is more efficient to produce energy by breaking down the monosaccharide glucose. Under these conditions, despite the presence of lactose, the transcription of the structural genes of the lac operon is suppressed. I will now explain this mechanism.

When glucose concentration decreases, the concentration of cyclic AMP (cAMP) increases. As glucose levels become low, adenylate cyclase produces cyclic AMP from ATP. Additionally, cells always contain Catabolite Activator Protein (CAP), which binds to cyclic AMP.

The cyclic AMP-CAP complex binds to specific DNA sequences and increases the transcription frequency of RNA polymerase. This phenomenon is utilized to regulate the transcription of structural genes in the lac operon.

In the transcriptional regulatory region of the lac operon, in addition to the operator region where the repressor binds, there is a CAP-binding site upstream of the promoter sequence.

The cyclic AMP-CAP complex can bind to this site. When the complex binds to the CAP site, RNA polymerase becomes able to stably bind to the promoter.

When glucose concentration is low, cyclic AMP concentration becomes high. As a result , the concentration of the cyclic AMP-CAP complex increases, leading to active transcription of the lac operon, which in turn promotes the breakdown of lactose in the culture medium.

On the other hand, when glucose concentration is high, cyclic AMP levels remain low, and the cyclic AMP-CAP complex is not formed. As a result, CAP cannot attach to the CAP-binding site.

In this state, the efficiency of RNA polymerase binding to the promoter is low.

When glucose concentration is high, cyclic AMP levels remain low, and the cyclic AMP-CAP complex is not formed. So, the transcription of the downstream lac structural genes becomes inactive. Consequently, even if lactose is present, its metabolism becomes inactive.

In this way, when monosaccharides like glucose and disaccharides like lactose coexist, we can see a very ingenious transcriptional regulation at work. The cell preferentially metabolizes glucose, which is easier to process, and only when glucose is depleted does it initiate the transcription of enzyme genes necessary for lactose metabolism.