Difference between RNA Polymerase I II and III

The synthesis of mRNA is a complex process in eukaryotes. In comparison to prokaryotes, where only a single polymerase is required to bind with the DNA strands, eukaryotes require three different polymerases, along with an additional set of transcriptional factors (TFs) for transcription and translation. The three polymerases required are: RNA polymerase I, RNA polymerase II and RNA polymerase III.

RNA Polymerase I

RNA Polymerase I, also known as Pol I is an enzyme that transcribes ribosomal RNA. It is composed of 14 subunits and weighs around 590 kDa. It transcribes the subunits of rRNA except the 5S rRNA subunit. The transcription by Pol I accounts for about 50% of the total transcription in a cell.

Refer: Structure, Functions and Types of RNA

RNA Polymerase II

RNA Polymerase II, also known as Pol II is an enzyme that transcribes all the precursors of mRNA, miRNA and snRNA. It is composed of 12 subunits and weighs 550kDa. It requires an assembly of transcription factors to begin transcription.

𝛂-Amanitin is a toxin produced by mushrooms that inhibits Pol II. It renders the enzyme totally unresponsive.

RNA Polymerase III

RNA Polymerase III, also known as Pol III transcribes tRNA and the 5S subunit of ribosomal RNA. It is composed of 17 subunits and weighs around 700 kDa. The genes transcribed by Pol III are directly in association with cell cycle and cell growth, therefore this enzyme needs to be active at all times and in every environment. Pol III does not require heavy assemblage of TFs like Pol II.

RNA Polymerase I

RNA Polymerase II

RNA Polymerase III


RNA Polymerase I is an enzyme that transcribes ribosomal RNAs.

RNA Polymerase II is an enzyme that transcribes precursors of mRNAs.

RNA Polymerase III is an enzyme that transcribes tRNAs.

Transcription Products

It transcribes all rRNAs except the 5S rRNA component.

It transcribes all the pre-mRNAs.

It transcribes tRNAs, 5S rRNA and snoRNAs.





Number of Subunits




Molecular Weight

590 kDa

550 kDa

Almost 700 kDa

Sensitivity to 𝛂-Amanitin

Not sensitive

Extra sensitive

Moderately sensitive

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Frequently Asked Questions

What is the difference between DNA Polymerases and RNA Polymerases?

DNA Polymerase is an enzyme that is required during the replication of DNA. Contrarily, RNA Polymerases are required for transcription and translation.

What are RNA Polymerases IV and V?

RNA Polymerase IV and V are found in plants and synthesise siRNA. They are less understood right now.

How many different RNA polymerases are found in prokaryotes?

There is only one RNA polymerase in prokaryotes.

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