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RNA Polymerase: Function and Definition

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Article

RNA Polymerase: Function and Definition

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Ribonucleic Acid (RNA) polymerase is an intermediary enzyme responsible for processing gene sequences into RNA-based genetic material that can be utilized in protein synthesis. In this article, we define RNA polymerase and further explore its various functions throughout cell biology.

What is RNA polymerase?


RNA polymerase is a multi-unit enzyme that synthesizes RNA molecules from a template of DNA through a process called transcription. The transcription of genetic information into RNA is the first step in gene expression that precedes translation, the process of decoding RNA into proteins.

RNA polymerase structure and function (in transcription)


The RNA polymerase enzyme is a large complex made up of multiple subunits1. The prokaryotic form of RNA polymerase has four subunits capable of transcribing all types of RNA. In eukaryotes, these enzymes have eight or more subunits that facilitate the attachment and processing of DNA throughout transcription.


RNA polymerase in action


The three stages of transcription involve various functions of RNA polymerase that result in the synthesis of RNA:

1. Initiation
begins when RNA polymerase wraps around the promoter region of DNA. The promoter is a DNA sequence that guides RNA polymerase on where to bind upstream of a gene. While prokaryotic RNA polymerase can directly bind to DNA promoter sequences, eukaryotic forms require the assistance of transcription factors for initial binding. Once RNA polymerase successfully binds DNA at the targeted promoter region, the enzyme can continue with the second stage of transcription.

2. Elongation
commences when RNA polymerase unwinds double-stranded DNA into two single strands. These DNA strands are used as genetic templates for RNA synthesis. As the DNA template strand moves through the RNA polymerase it builds an RNA strand that is complimentary to the transcribed DNA strand.

3. Termination
is the final step of transcription. Once RNA polymerase encounters a terminator sequence or signal, it stops adding complementary nucleotides to the RNA strand. This is followed by the release of the RNA transcript, which marks the end of transcription for that template of DNA.

What are the different types of RNA polymerase?


While prokaryotes like bacteria have one RNA polymerase that transcribes all types of RNA, eukaryotes like plants and mammals can have numerous forms of RNA polymerase.

RNA polymerase I


RNA polymerase I2 is responsible for synthesizing most ribosomal RNA (rRNA) transcripts. These transcripts are produced within the nucleolus, a region within the nucleus where ribosomes are assembled. The availability of rRNA molecules produced by RNA polymerase can impact essential functions of cell biology since these transcripts are directly involved with the production of ribosomes.

RNA polymerase II


RNA polymerase II3 transcribes protein-coding genes into messenger RNA (mRNA). This 12-subunit enzyme works as a complex that directly influences gene expression through its production of pre-mRNA transcripts. Once the pre-mRNAs are released by RNA polymerase II within the nucleus, biochemical modifications prepare these transcripts for translation. RNA polymerase II also produces micro RNA (miRNA) molecules. These non-coding transcripts can mediate gene expression and the activity of mRNAs after transcription.

RNA polymerase III


RNA polymerase III2 transcribes rRNA genes into small RNAs like transfer RNA (tRNA) and 5S rRNA. These smaller RNA transcripts play a role in normal cell function throughout the nucleus and cytoplasm.

RNA polymerase IV and V


Exclusively found in plants, RNA polymerase IV and V are transcription enzymes that evolved as specialized forms of RNA polymerase II4. Both enzymes produce small interfering RNA (siRNA) transcripts, which play a role in the silencing of plant genes.

RNA polymerase vs DNA polymerase


DNA polymerase synthesizes double-stranded DNA molecules from unwound DNA strands during replication. Even though the end products of replication and transcription are different, they both work upon DNA by adding nucleotides in the same 5’ to 3’ direction. In contrast with RNA polymerase, DNA Polymerase is semi-conserved process that utilizes both strands of a double-stranded DNA molecule as a template for replication.

Comparison

RNA Polymerase

DNA Polymerase

Function

Transcription of DNA

Replication of DNA

Purpose

To make RNA copies of genes

To copy the entire genome

Time of occurrence

Used in transcription during G phase(s)

Used in replication during S phase

Primer

Not required for transcription

Required for initiation of replication

Base pairs used to synthesize product

Adenine, Guanine, Cytosine and Uracil

Adenine, Guanine, Cytosine and Thymine

Resulting product

Single-stranded RNAs (e.g. mRNA)

Double-stranded DNAs


RNA polymerase and drugs


RNA polymerase is an attractive target for drug development due to its ubiquitous presence and function throughout life. The biochemical differences in RNA polymerase between prokaryotes and eukaryotes allow for specific drugs that target microbial RNA polymerases without any interaction with our own.

Several antimicrobial drugs function as RNA polymerase inhibitors by blocking bacterial or viral enzyme activity during one stage of transcription. For example, the rifamycins5 are a group of bacterial antibiotics that inhibit elongation by blocking the exit channel of RNA polymerase. These drugs are commonly used to treat challenging infections caused by leprosy and tuberculosis.


References:

1.      Mooney RA, Landick R. RNA polymerase unveiled. Cell. 1999;98(6):687-690. doi:10.1016/s0092-8674(00)81483-x.

2.      Khatter H, Vorländer MK, Müller CW. RNA polymerase I and III: similar yet unique. Curr Opin Struct Biol. 2017;47:88-94. doi:10.1016/j.sbi.2017.05.008.

3.      Schier AC, Taatjes DJ. Structure and mechanism of the RNA polymerase II transcription machinery. Genes Dev. 2020;34(7-8):465-488. doi:10.1101/gad.335679.119.

4.      McKinlay A, Podicheti R, Wendte JM, Cocklin R, Rusch DB. RNA polymerases IV and V influence the 3' boundaries of Polymerase II transcription units in Arabidopsis. RNA Biol. 2018;15(2):269-279. doi:10.1080/15476286.2017.1409930.

5.      Sensi P (1983). "History of the development of rifampin". Reviews of Infectious Diseases. 5 Suppl 3: S402–6. doi:10.1093/clinids/5.supplement_3.s402. PMID 6635432

Meet The Author
Jonathan Dornell, PhD
Jonathan Dornell, PhD
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