Difference Between Operon and Regulon

Key Difference – Operon vs Regulon
 

The operon is a functional DNA unit in prokaryotes consists of several genes that are regulated by a single promoter and an operator. Regulon is a functional genetic unit that is composed of a noncontiguous group of genes regulated by a single regulatory molecule. The key difference between the Operon and the Regulon is the contiguous or noncontiguous nature of genes. Gene cluster of an operon is contiguously located while the genes of a regulon can be located noncontiguously.

Regulation of gene expression in prokaryotes and eukaryotes takes place with the use of different mechanisms. Prokaryotes use the concept of operon to regulate their gene expression while eukaryotes use the concept of a regulon for their gene regulation.

CONTENTS

1. Overview and Key Difference
2. What is an Operon
3. What is a Regulon
4. Similarities Between Operon and Regulon
5. Side by Side Comparison – Operon vs Regulon in Tabular Form
6. Summary

What is an Operon?

Operons are predominantly and primarily found in prokaryotes, although there are very recent discoveries where operons were seen in some eukaryotes including nematodes (C. elegans). An operon is composed of several genes that are regulated by a common promoter and a common operator. The operon is regulated by repressors and inducers. Thus, the operons can be mainly classified as inducible operons and repressible operons. Therefore, as the operon consists of multiple genes, it gives rise to a polycistronic mRNA upon the completion of transcription.

There are two main operons studied in prokaryotes; the inducible Lac operon and the repressible Trp operon. The structure of an operon is typically studied with respect to the lac operon. The lac operon is composed of a promoter, operator and three genes namely Lac Z, Lac Y and Lac A. These three genes code for three enzymes that are involved in lactose metabolism in microbes. Lac Z codes for Beta-galactosidase, Lac Y codes for Beta – galactoside permease and Lac A codes for Beta – galactoside transacetylase. All three enzymes help in the degradation and transportation of lactose. Thus, in the presence of lactose, the compound allolactose is formed that binds to the lac repressor allowing RNA polymerase action to proceed and result in the transcription of the genes. In the absence of lactose, the lac repressor is bound to the operator, thereby blocking the activity of RNA polymerase. Thus, no mRNA is synthesized. Thus, the lac operon acts as an inducible operon, where the operon is functional when the substrate lactose is present.

In comparison, the trp operon is a repressible operon. Trp operon codes for five enzymes required in the synthesis of tryptophan that is an essential amino acid. Thus, the activity of trp operon is active all the time. When there is an excess of tryptophan, the operon is inhibited, thus known as a repressible operon. This will result in the inhibition of tryptophan production until a homeostatic condition is reached.

Figure 01: Operon

Therefore, both lac operon and the trp operon is involved in gene regulation and thereby, participate in conserving cells energy and maintaining the accuracy of cellular activities at a molecular level.

What is a Regulon?

Regulons, were previously identified in bacteria as well, where a cluster of operons named as a regulon. At present, a regulon is a DNA fragment or a genetic unit that is under the control of a common regulatory gene. Therefore, more than the promoter and the operator, a new regulator gene is involved in regulon gene expression. This is now observed predominantly in eukaryotes. The genetic unit is composed of a noncontiguous group of genes. Therefore, these genes are not placed in a specific, definite order and can be distributed throughout the genome of the eukaryotes.

Figure 02: Regulon

In prokaryotic bacteria, Regulon is referred to as a bunch operons operating together. A Regulon is mainly categorized as a modulon or a stimulon. A modulon responds to all types of stresses and conditions, whereas a stimulon only responds to environmental changes or stimuli. The prokaryotic examples of Regulon are observed in phosphate regulation and in the regulation of responses to heat shock stresses via sigma factors. In eukaryotes, these regulons are involved in controlling translation via the binding of translation factors that are either induce or inhibit the translation process in eukaryotes.

What are the Similarities Between Operon and Regulon?

• Both Operon and Regulon are involved in the regulation of gene expression.
• Both Operon and Regulon are composed of DNA.
• Both Operon and Regulon are regulated by inducers, repressors or stimulators.

What is the Difference Between Operon and Regulon?

Summary – Operon vs Regulon

Operons are Regulons involve in the regulation of gene expression. Although both of these regulatory mechanisms were observed in prokaryotes initially, regulons were then found to predominantly present in eukaryotes. They were found to have a regulatory role in the eukaryotic gene transcription and translation. Operons are mainly either inducible or repressible. They are composed of a group of genes containing a single promoter and a single operator, whereas, in the regulon, a regulatory gene is involved in controlling a set of noncontiguous genes in eukaryotes. This is the difference between operon and regulon.

Reference:

1.Culjkovic, B, et al. “Controlling Gene Expression through RNA Regulons: the Role of the Eukaryotic Translation Initiation Factor eIF4E.” Cell Cycle (Georgetown, Tex.)., U.S. National Library of Medicine, 1 Jan. 2007. Available here 
2.“Gene Regulation: Operon Theory.” Lumen. Available here 

Image Courtesy:

1.’Lac-operon’By Barbarossa at Dutch Wikipedia, (CC BY-SA 3.0) via Commons Wikimedia  
2.’NIF REGULON’By Bt09b020 – Own work, (CC BY-SA 3.0) via Commons Wikimedia

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