Table of Contents
What is RNA Polymerase?
RNA Polymerase, abbreviated as RNA Pol or RNAP, is an enzyme in molecular biology that synthesises RNA from a DNA template. During the process of transcription, RNA polymerase copies the sequence of DNA into an RNA sequence with the help of the enzyme helicase that opens up the wounded DNA strands.
The RNA Pol not only transcribes DNA but also facilitates the process of attachment and elongation of nucleotides, proofreads the transcribed RNA and takes part in the recognition of terminator regions.
The RNA produced by RNAP are functional mRNAs that encode protein (translation) or produce non-coding functional RNAs, such as tRNA, rRNA and miRNA. RNA Polymerase is an important enzyme that is found in prokaryotes, eukaryotes, as well as viruses. The size and number of subunits in the RNAP complex are variable depending upon the type of organism.
While bacteria and archaea have a single RNA Pol, eukaryotes have five types of enzyme, each with their designated functions.
Structure of RNA Polymerase
Roger D. Kornberg, an American biochemist, demonstrated a detailed molecular image of the RNAP enzyme during various transcriptional stages. He was awarded a Nobel Prize for the demonstration.
In prokaryotes, the single type of RNA Polymerase enzyme is composed of a core of five subunits: two alpha subunits (36 kDa), one beta subunit (150 kDa), one beta prime subunit (155 kDa) and a small-sized omega subunit. The core enzyme binds with a sigma factor to form a holoenzyme. The core enzyme forms a crab claw along the length of the DNA to be transcribed. The RNA Pol enzymes contain metal cofactors, such as zinc and magnesium, that help in the process of transcription.
The eukaryotic RNAPs have a similar core structure to the enzyme but also have some additional subunits.
Function of RNA Polymerase
RNAP is an important enzyme that initiates transcription in both prokaryotes and eukaryotes. It induces specific DNA sequences on the DNA strand known as a promoter to initiate transcription.
RNA Pol can produce complementary RNA chains to the template DNA strand and can add up to 2.4 million nucleotides in eukaryotes in a process called elongation.
RNA polymerase can produce the following RNA products:
- mRNA (messenger RNA) that translates into proteins.
- RNA genes or non-coding RNA that produce RNA strands, which do not encode proteins but take part in other activities. They include:
- tRNA (transfer RNA) that adds amino acids to the growing polypeptide chain in the process of translation.
- rRNA (ribosomal RNA) is a component of the ribosomes that takes part in translation.
- miRNA (microRNA) regulates gene activity.
- Ribozymes are enzymatically active RNA molecules.
Components of RNA Polymerase
Bacteria
In bacteria, the RNAP enzymes produce both mRNA and non-coding RNA. The enzyme is a large molecule and consists of 5 subunits:
- β’: It is the largest subunit and consists of parts active centre that is responsible for RNA synthesis.
- β: It is the second largest subunit that consists of the remaining portion of the active centre.
- ɑ: It is the third largest subunit that is present in two copies: ɑI and ɑII.
- ⍵: It is the smallest subunit that facilitates the assembly of RNA Pol enzyme.
The RNA Pol binds with the σ factor (transcription initiation factor) to form the RNAP holoenzyme. This reduces the affinity of RNAP for non-specific DNA binding and increases its binding affinity for promoters.
Eukaryotes
In eukaryotes, there are many types of RNA Pol enzymes that have different functions:
- RNA Pol I: It synthesises pre-rRNA subunits that form the ribosomes.
- RNA Pol II: It synthesises precursors of microRNA, snRNA and mRNA.
- RNA Pol III: It synthesises tRNA, other precursors of rRNA and some small RNAs.
- RNA Pol IV: It is found in plants and synthesises siRNA.
- RNA Pol V: It is also involved in the synthesis of siRNa in plants.
Archaea
Archaea has one single type of RNAP that synthesises all RNAs. Structurally, it is very similar to bacterial and eukaryotic RNAP, specifically RNA Pol II.
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