Difference Between Promoter and Operator

Key Difference – Promoter vs Operator
 

DNA sequences other than the coding region of a gene are vital in performing various functions in relation to the process of transcription. Transcription is the enzyme-catalyzed process that transcribes or converts the DNA strand into its similar mRNA strand. In the central dogma of life, transcription of DNA to mRNA is the first phase of protein synthesis. This is followed by translation, which converts the mRNA sequence into an amino acid sequence that makes the expected protein. Among the different sequences found in organisms, promoter sequences and operator sequences play a major role in transcription. Promoters are present in both prokaryotes and eukaryotes. They are positioned upstream of the transcription start site and are the sites in which RNA polymerase enzyme binds. Operators are only present in prokaryotes. They are the sites in which the regulatory molecule binds to an operon. The key difference between the promoter and the operator is based on the type of molecule that binds to the respective DNA sequence. RNA polymerase binds to the promoter, whereas regulatory molecules of the operon system bind to the operator.

CONTENTS

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

What is a Promoter?

A promoter is a DNA sequence placed upstream of the transcription start site. This important DNA sequence is found in both eukaryotes and the prokaryotes; although the eukaryotic promoters may differ from the prokaryotic promoters. Promoters are the DNA regions in which RNA polymerase binds to during the transcription process. It is the main enzyme involved in producing the single-stranded RNA (mRNA, tRNA, rRNA) from a DNA template. Depending on the type of RNA, the RNA polymerase will differ. The promoter sequences are highly conserved regions throughout the genome. Therefore, they are known as consensus regions. The mechanism of action of the promoter differs in eukaryotes and prokaryotes.

In eukaryotes, the conserved sequence found in the promoters is termed as the TATA box, which is located at the -10 position of the gene. The binding of RNA polymerase to the TATA box is facilitated by the binding transcription factors. This transcription factors, make confirmation changes in the promoter sequence and increase its affinity for RNA polymerase to bind. Thus the pre-initiation complex formed during transcription initiation is composed of the complex formed with the 7 transcription factors and the promoter site. Once this complex is formed, eukaryotic RNA polymerase readily binds to the promoter and initiates the transcription.

In prokaryotes, the mechanism is much simpler as they do not contain any transcription factors. Instead, the sigma factor of RNA polymerase is involved in recognizing the promoter and in the assembly of the enzyme on the promoter. There are two main conserved promoter regions in prokaryotes, the corresponding promoter sequence to the TATA box is known as the “Pribnow Box”. The Pribnow Box (-10 position) is composed of the sequence TATAAT. The second promoter sequence is known as the -35 element as it is located at the -35 position.

What is an Operator?

An operator is found in prokaryotic gene structure. It is the main region of DNA in which the regulatory molecules of an operon system binds to. The lac operator is the operator sequence present in the lac operon of many prokaryotic bacteria. In the case of the lac operon, the repressor molecule binds to the operator region. This binding will prevent RNA polymerase from transcribing the genes present downstream of the operator.

Eukaryotes do not possess operator regions. Instead, their transcription factors involved in regulation of transcription are bind to the promoter regions. Thus, the main function of the operator in prokaryotes is to regulate gene expression.

What are the Similarities Between Promoter and Operator?

• Both Promoter and Operator are composed of deoxyribose nucleic acids (DNA).
• Both Promoter and Operator sequences are important in the transcription process.

What is the Difference Between Promoter and Operator?

Promoter vs Operator
Promoters are the sites in which RNA polymerase binds and they are present upstream of the transcription start site of a gene.	Operators are the sites in which the regulatory molecule binds into an operon model.
Type of Organism
Promoters are found in both prokaryotes and eukaryotes.	Operators are found only in prokaryotes.
Function
Promoter facilitates the binding of the RNA polymerase and transcription factors (only in eukaryotes) to the gene for gene transcription. In prokaryotes, promoter region facilitates the binding of sigma factor of RNA Polymerase (in prokaryotes).	Operators regulate the gene expression by facilitating the binding of the regulatory molecule to the operon.

Summary – Promoter vs Operator

Promoter and Operator are important DNA sequences that are involved in the transcription process and in transcription regulation. Promoter sequences are found in both prokaryotes and eukaryote. Promoter is the site for binding of RNA polymerase. They are highly conserved regions known as consensus sequences. The TATA box of eukaryotes and the Pribnow box and the -35 promoter of prokaryotes are the common promoters. Operators are only present in prokaryotes, where they control the gene expression by binding the repressor and inhibiting transcription of the downstream genes (lac operon concept), or bind to the activator and induce transcription (trp operon concept). This is the difference between promoter and operator.

Reference:

1.“BioCoach Activity.” Pearson – The Biology Place, Available here 
2.“Promoters.” Addgene, Available here  

Image  Courtesy:

1.’Figure 16 02 02’By CNX OpenStax , (CC BY 4.0) via Commons Wikimedia
2.’Lac-operon’By Barbarossa at Dutch Wikipedia, (CC BY-SA 3.0) via Commons Wikimedia