The main difference between lac operon and arabinose operon is that the Lac operon is negatively regulated whereas the arabinose operon is both positively and negatively regulated according to circumstances.
Lac operon and arabinose operon are two types of operons that occur in prokaryotes. They are clusters of genes that give rise to a polycistronic mRNA. These genes are responsible for the metabolism.
Key Areas Covered
1. What is Lac Operon
– Definition, Structure, Function
2. What is Arabinose Operon
– Definition, Structure, Function
3. Similarities Between Lac Operon and Arabinose Operon
– Outline of Common Features
4. Difference Between Lac Operon and Arabinose Operon
– Comparison of Key Differences
Key Terms
Arabinose Operon, AraBAD Operon, Lac Operon
What is Lac Operon
The lac operon refers to a cluster of genes responsible for lactose metabolism in 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. Moreover, lacl is the regulatory gene of the lac operon, and its gene products prevent the binding of RNA polymerase to the promoter. Meanwhile, the cAMP-bound catabolite activator protein (CAP) is the protein that aids the binding of RNA polymerase into the promoter of the lac operon.
Figure 1: The Lac Operon
Furthermore, the lac operon consists of three structural genes controlled by a single promoter. They are lacZ, lacY, and lacA. These genes are encoded for the three enzymes responsible for lactose metabolism known as beta-galactosidase, beta-galactoside permease, and beta-galactoside transacetylase, respectively. Beta-galactosidase is responsible for 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 responsible for 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 three gene products from a single mRNA molecule.
What is Arabinose Operon
The Arabinose operon is the gene cluster responsible for the metabolism of arabinose. Other names for arabinose operon are ara or araBAD operon. Most importantly, the arabinose operon consists of three structural genes: araB, araA, and araD. However, araB is encoded for ribulokinase enzyme, while araA is encoded for isomerase, and araD is encoded for epimerase enzyme. By the enzymes, arabinose is first converted into ribulose by the arabinose isomerase. Then, ribulose is phosphorylated by ribulokinase. Finally, it is converted into xylulose 5-phosphate by the enzyme ribulose 5-phosphate epimerase. The final product, xylulose 5-phosphate, enters the pentose phosphate pathway.
Figure 2: The Arabinose Operon
Moreover, the promoter of the arabinose operon is PBAD. When arabinose is absent, the AraC protein is synthesized, and it binds to the araC gene. Then, AraC serves as its own regulator. This prevents the binding of RNA polymerase into the promoter, preventing transcription of the arabinose operon. However, in the presence of arabinose, AraC makes structural changes in the operon to bind RNA polymerase to the promoter.
Similarities Between Lac Operon and Arabinose Operon
- Lac operon and arabinose operon are two types of gene clusters that occur in prokaryotes.
- They are important for the metabolism of organic compounds inside bacteria.
- They produce polycistronic mRNA.
Difference Between Lac Operon and Arabinose Operon
Definition
Lac operon refers to an operon or a group of genes with a single promoter that encodes genes for the transport and metabolism of lactose in E.coli and other bacteria, while arabinose operon refers to the system carrying the number of genes like a regulatory, promoter, operator, inducer, and structural genes for L-Arabinose’s breakdown into xylulose 5-phosphate.
Type of Metabolism
The lac operon is responsible for the metabolism of lactose, while the arabinose operon is responsible for the metabolism of arabinose.
Regulation
Moreover, the lac operon is negatively regulated while the arabinose operon is both positively and negatively regulated according to the circumstances.
Structural Genes
The lac operon consists of three structural genes: lacZ, lacY, and lacA while arabinose operon consists of three structural genes: araB, araA, and araD.
Regulatory Gene
In addition, the regulatory gene of the lac operon is lacl while the regulatory gene of the arabinose operon is araC.
Mutations
The mutations of the lacl genes cause lac operon to express constitutively, while the mutations of the araC gene cause arabinose operon to be super repressed even though arabinose is abundant.
Constitutive Mutations
Furthermore, constitutive mutations are common in the lac operon while constitutive mutations are rare in the arabinose operon.
Conclusion
In brief, lac operon and arabinose operon are two types of operons that occur in prokaryotes. The lac operon is responsible for the breakdown of lactose. It is a negatively regulated operon. It contains three structural genes: lacZ, lacY, and lacA, and one regulatory gene: lacl. However, the lac operon contains common constitutive mutations. In comparison, arabinose operon is responsible for the breakdown of arabinose. Arabinose operon is both negatively and positively regulated. Moreover, the three structural genes of arabinose operon are araB, araA, and araD and the regulatory gene is araC. In addition, constitutive mutations are rare in arabinose operons. Therefore, the main difference between the lac operon and the arabinose operon is their regulation.
References:
- “Lac Operon -An overview.”ScienceDirect Topics.
- “The ARA operon.” -University of Pennsylvania.
Image Courtesy:
- “Figure 16 02 02” By CNX OpenStax – Own work (CC-BY 4.0) via Commons Wikimedia
- “Structure of L-arabinose operon of E. coli” By Yiktingg1 – Own Work (CC-BY SA 4.0) via Commons Wikimedia
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