What is the Difference Between Sigma Factor and Rho Factor

The key difference between sigma factor and rho factor is that sigma factor is a protein needed for the initiation of transcription in bacteria while rho factor is a protein needed for the termination of transcription in bacteria.

Sigma factor and rho factor are two important factors for bacterial transcription. Bacterial transcription is a process in which a piece of bacterial DNA is copied into a newly made strand of mRNA (messenger RNA). This process is catalyzed by an enzyme known as RNA polymerase. Bacteria heavily rely on transcription and translation in order to respond to their environment properly. The bacterial RNA polymerase consists of a core and a holoenzyme structure. The core contains the subunits β, β′, α and 2ω while the holoenzyme contains the sigma factor. Moreover, an ancillary factor to RNA polymerase known as the rho factor is also very important for bacterial transcription.

CONTENTS

1. Overview and Key Difference
2. What is Sigma Factor
3. What is Rho Factor
4. Similarities – Sigma Factor and Rho Factor
5. Sigma Factor vs Rho Factor in Tabular Form
6. Summary – Sigma Factor and Rho Factor

What is Sigma Factor?

Sigma factor or specificity factor is a protein needed for the initiation of transcription in bacteria. It is a bacterial transcription initiation factor. This factor enables the binding of RNA polymerase enzyme to a specific region of DNA called the promoter region during transcription. Sigma factor is normally homologous to archaeal transcription factor B and to eukaryotic transcription factor TF11B. Moreover, the specific sigma factor of a given gene varies depending on the gene and environmental signals. The promoters are selected by RNA polymerase depending on the sigma factor associated with it.

The sigma factor can also be found in the chloroplast of plants as a part of the plastid-encoded polymerase (PEP). The sigma factor and RNA polymerase altogether is known as RNA polymerase holoenzyme. The molecular weight of the sigma factor is around 70 kDa. After the initiation of transcription, the sigma factor dissociates from the complex. Then the RNA polymerase continues elongation till transcription terminates.

What is Rho Factor?

Rho factor is a prokaryotic protein involved in the termination of transcription. Rho factor usually binds to the transcription terminator pause site. This site is an exposed region of the single-stranded RNA near the open reading frame at cytosine rich guanine poor sequences. It is an essential transcription protein in prokaryotes. In Escherichia coli, the rho factor is approximately 274.6 kDa hexamer of identical subunits. Each subunit has an RNA binding domain and ATP hydrolysis domain. Moreover, the function of the rho factor is helicase activity.

Rho factor binds to RNA molecule and then uses its ATPase activity to provide energy to translocate RNA molecules. Rho factor carries out translocation until it reaches RNA-DNA helical region, where it unwinds the hybrid (RNA-DNA) duplex structure. This process pauses RNA polymerase at the termination site terminating the transcription process.

What are the Similarities Between Sigma Factor and Rho Factor?

• Sigma factor and rho factor are two important factors for bacterial transcription.
• Both are present only in prokaryotes.
• They are proteins made up of amino acids.
• Both are nucleic acids binding proteins.
• Prokaryotes heavily rely on these factors to generate mRNA that can make peculiar proteins and help them respond to their environment specifically.

What is the Difference Between Sigma Factor and Rho Factor?

Sigma factor is a protein needed for initiation of transcription in bacteria, while rho factor is a protein needed for termination of transcription in bacteria. So, this is the key difference between sigma factor and rho factor. Furthermore, the sigma factor is a DNA binding protein, while the rho factor is an RNA binding protein.

The below infographic presents the differences between sigma factor and rho factor in tabular form for side by side comparison.

Summary – Sigma Factor vs Rho Factor

Transcription is the process where the information in DNA is copied into a new molecule called mRNA (messenger RNA). This m-RNA is ultimately involved in producing a specific protein. Sigma factor and rho factor are two important factors for bacterial transcription. They are proteins. Sigma factor participates in the initiation of transcription in bacteria. Rho factor is responsible for the termination of transcription in bacteria. Thus, this is the summary of the difference between sigma factor and rho factor.

Reference:

1. “Sigma Factor.” An Overview | ScienceDirect Topics.
2. “Rho Factor.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Anti-sigma factor” By Halah.F – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Rho And Intrinsic Termination photo” By Jeffrey W Roberts – (CC BY-SA 4.0) via Commons Wikimedia