Introduction
The lac operon is a gene regulation system in the bacterium Escherichia coli (E. coli) that allows lactose, a sugar found in milk, to be metabolised efficiently. These include the lacZ, lacY and lacA genes that code for beta-galactosidase, permease and transacetylase respectively. For the breakdown of lactose into simpler sugars and its transport through the cell membrane, these enzymes are also indispensable.
A promoter, operator and a regulatory gene (lacI) which produces the repressor protein for the lac operon tightly control its expression. If there is no lactose present, then repressor binds with operator hence blocking transcription process of operons genes. The presence of lactose makes it possible for it to bind to the repressor such that its shape changes making it unable to connect with operator thereby enabling transcription to take place.
Additionally, when glucose levels are low, cyclic AMP (cAMP) rises and cAMP-CAP complex increases RNA polymerase association with promoters which stimulates their transcription activity. In this way E.coli saves energy by synthesizing necessary enzymes for metabolism of lactose only when glucose is absent but not if glucose is available thus indicating intelligent mechanisms directing resource use efficiency among bacteria.
What is an Operon?
An operon refers to a group of genes within which transcriptions lead to the formation of a single mRNA (messenger RNA) molecule. Such genes are regulated by a single promoter, repressor, and operator.
Operons may be classified as either inducible or repressible:
- Inducible
- This type of operon can be activated when an inducer molecule is present such as in lac and arabinose operons.
- Repressible
- Their characteristics include being associated with anabolic pathways.
- Where the cell is operating normally, the operon will continue to remain on and hence their products appearing in free form.
- This process ceases upon reaching the maximum desired end product concentrations eg trp operon
Lactose Operon (Lac operon)
The Lac Operon is simply a cluster of related genes that have one promotor so that they are seen as one gene when transcribed into one mRNA molecule.
Within this operon are enzymes that bring about lactose metabolism in bacteria cells.
Even though E.coli bacteria can break down lactose, it does not like it too much because glucose is just around somewhere and they would preferentially utilize glucose instead. The catabolism of lactose requires more steps than glucose breakdown.. Nevertheless, if lactose were the only sugar available, then E.coli would use it for its energy needs immediately after its uptake since glucose is broken down faster than lactose by coli
For E. coli to express lac operon, two conditions need to be fulfilled:
- When lactose is available
- When glucose is absent.
Structure of lac operon
lac Z – The gene for β-galactosidase which codes for the enzyme that breaks down lactose into glucose and galactose.
lac Y – The gene for β-galactoside permease, a membrane protein that transports lactose into the cell.
lac A – Another gene in the lac operon is β-Galactoside transacetylase which catalyzes the transfer of the acetyl group from acetyl-CoA to -β galactosides, glucosides and lactosides).
Apart from these genes, there are also regulatory DNA sequences in the lac operon. These are regions of DNA at which specific regulatory proteins can attach so as to control transcription of this operon.
- The promoter will bind with RNA polymerase during transcription.
- One negative regulator site on the operator binds a repressor protein called Lac repressor. The operator overlaps the promoter region such that when the repressor binds RNA polymerase cannot initiate transcription here.
- Another positive regulator site occupies the CAP binding site where Catabolite Activator Protein (CAP) will attach itself promoting transcription by facilitating RNA polymerase’s contact with the promoter.
Along with these sequences, there is one more promoter before the operon sequence.
- PI – Promoter for Lac I
- Lac I – Codes for the protein I (repressor) that will bind to the operator region and represses the lac operon. It is actually an inhibitor of the lac operon.
Lac I is continuously expressed regardless of the presence or absence of lactose.
- When Glucose is present, Lac I (repressor) will bind to operator region and will inhibit the expression of Lac operon.
- When Lactose is present, a small amount of lactose enters the cell. Here it gets converted into Allolactose. Allolactose then binds with Lac I. Conformational change takes place which looses Lac I connection with the operator region. Now the operator region is free. RNA polymerase can bind to the promoter region and transcribe all genes.
There is a some sequence before the promoter region known as the CAP binding site (Catabolic Activator Protein).
When the Glucose in the cell is high, the cAMP is low. (Glu ↑ cAMP ↓)
When the Glucose in the cell is low, the cAMP is high. (Glu ↓ cAMP ↑)
cAMP and CAP form a complex and bind to the CAP binding site.
For the expression of the Lac operon, 2 signals are required:
- Presence of Lactose
- Depletion of Glucose (Hence cAMP is ↑)
Cases:
- Absence of lactose and presence of glucose: This is because the operator remains bound to the lac repressor, thus interfering with transcription by RNA polymerase. Furthermore, due to high levels of glucose there is low amount if cAMP, which in turn results in incapability of CAP activity affecting the binding DNA.
- Presence of both glucose and lactose: The occurrence of low-level transcription results from this. The inducer (allolactose) being present causes the detachment of the lac repressor from the operator. There are however reduced cAMP levels since there’s presence of glucose. Therefore, CAP fails to be active hence incapable of binding DNA resulting in transcription at very low or weak rate referred to as leaky.
- Absence of Glucose and Lactose: High-level production does not happen in this case because CAP is released due to lack or deficiency of allolactose while cAMP continues increasing when glucose decreases thereby leading to its binding with the DNA. However, other than that lac repressor will also be bound to the operator acting like a roadblock against RNAP hindering it from initiating any transcription because allolactose isn’t available.
- Glucose absent, lactose present: Glucose is not present but lactose is there hence strong level of transcription will occur. Due to the presence of allolactose as an inducer, the operator releases the lac repressor. The absence of glucose causes high levels of cAMP and so CAP binds to DNA. High levels of transcription are possible because CAP allows RNA polymerase to attach to a promoter.
References
- https://www.khanacademy.org/science/ap-biology/gene-expression-and-regulation/regulation-of-gene-expression-and-cell-specialization/a/the-lac-operon
- https://byjus.com/biology/lac-operon-regulation-gene-expression/
- James Watson, Molecular Biology of the Gene, 7th
- Robert Weaver, Molecular Biology, 5th
