What is the Difference Between Operon and Regulon

The main difference between operon and regulon is that the genes in an operon occur in the genome contiguously whereas the genes in a regulon occur in different locations within the genome. Furthermore, an operon consists of a set of functionally-related genes while a regulon may consist of several operons. 

Operon and regulon are two types of gene clusters in the prokaryotic genome. A common regulatory mechanism is responsible for the regulation of each type of gene clusters. 

Key Areas Covered 

1. What is an Operon
     – Definition, Type of Clusters, Importance
2. What is a Regulon
     – Definition, Type of Clusters, Examples
3. What are the Similarities Between Operon and Regulon
     – Outline of Common Features
4. What is the Difference Between Operon and Regulon
     – Comparison of Key Differences

Key Terms 

Cluster of Genes, Operon, Prokaryotic Genome Structure, Regulatory Mechanisms, Regulon

What is an Operon 

An operon is a group of genes whose expression is regulated coordinately. Most significantly, the genes in the operon occur contiguously in the genome. Therefore, the transcription of an operon results in a single mRNA molecule, which is polycistronic. This means, all the genes in an operon are transcribed into a single mRNA molecule. Therefore, the translation of this polycistronic mRNA produces several proteins that are functionally-related.  

Figure 1: Structural Elements of the Lac Operon

The main importance of the clustering of the functionally-related genes into operons is the ability to regulate this gene group together under a single promoter. This is why they are transcribed together. Also, a particular operon consists of a single enhancer or repressor. Thus, a particular operon can be either inducible or repressible. The Lac operon, which is an inducible operon, and Trp operon, which is a repressible operon, are such much-studied operons in the prokaryotic genome. 

What is a Regulon 

A regulon is a group of genes or operons regulated by a single regulatory mechanism. The factor responsible for this regulation is a transcription factor, which is a regulatory protein. Here, these operons or genes occur in different locations in the genome. But, each of them contains a common regulatory element binding site or a promoter, which helps the simultaneous regulation of the gene expression. Significantly, all the gene products of these operons are functionally-related.  

Figure 2: Structural Elements of the Regulon for the SOS Response

The genes responsible for arginine biosynthesis in prokaryotes form a regulon. It consists of 9 genes, clustered into 2 operons and 3 single genes. Each of the 5 transcription units contains individual promoters. But, they share a common operator, which is a reprosser. Another regulon in prokaryotes responsible for the SOS response expressed in the presence of a DNA damage.  

A modulon refers to a set of regulons capable of producing changes in response to stress. However, since this involves several regulons, several regulatory mechanisms are employed in the regulation of the response. On the other hand, a stimulon refers to a set of genes expressed in response to the specific environmental stimuli. 

Similarities Between Operon and Regulon 

• Operon and regulon are two types of genome structures in the prokaryotic genome. 
• Both consist of clusters of genes regulated by a single regulatory mechanism. Both systems share a common operator.  
• Also, both produce functionally-related gene products.  

Difference Between Operon and Regulon 

Definition 

Operon refers to a unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together while a regulon refers to a group of genes regulated by the same regulatory molecule and may be located non-contiguously in the genome. This contains the main difference between operon and regulon.

Occurrence 

Where they occur attributes to a difference between operon and regulon. That is; the operons occur in the prokaryotic genome while the regulons occur in both prokaryotic and eukaryotic genomes. 

Location 

We can also identify a difference between operon and regulon based on the location. The genes of an operon occur contiguously in a particular location of the genome while the genes of a regulon occur in different locations within the genome.  

Significance 

Another difference between operon and regulon is that an operon consists of a set of functionally-related genes while a regulon may consist of several operons or multiple genes. 

Regulation 

Moreover, an operon is coregulated while a single transcription factor regulates a regulon.  

Promoter 

An operon is regulated under a single promoter while each transcription unit of a regulon comprises an individual promoter. Hence, this is another difference between operon and regulon.

mRNA 

Furthermore, an operon produces a single, polycistronic mRNA by transcription while each transcription unit of a regulon produces separate mRNAs, which can be either polycistronic or monocistronic. 

Examples 

Lac operon and Trp operon are two examples of operons while some of the prokaryotic operons are regulons responsible for the arginine biosynthesis and SOS response. 

Conclusion 

An operon is a cluster of genes regulated under a single promoter. These genes occur in a contiguous manner in the genome. Hence, it produces a polycistronic mRNA molecule by transcription. On the other hand, a regulon is a group of operons or genes. Here, each transcriptional unit is under an individual promoter and they are located in different locations in the genome. In the meantime, all the gene products of an operon or a regulon are functionally-related. In brief, the main difference between operon and regulon is the location in the genome and the type of transcriptional unit.  

Reference:

1. “Gene Regulation: Operon Theory.” Lumen Microbiology, Openstax|Lumnen Learning, Available Here
2. Zhang, Han, et al. “Genomic Arrangement of Regulons in Bacterial Genomes.” PLOS ONE, Public Library of Science, 3 Jan. 2012, Available Here

Image Courtesy:

1. “Lac operon-2010-21-01” By Lac_operon.png: G3proderivative work: Tereseik (talk) – Lac_operon.png (CC BY 2.0) via Commons Wikimedia  
2. “Sos response 2” By Ragavi , Prathyusha – Created on own (Public Domain) via Commons Wikimedia