RNA polymerase

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RNA polymerase is an enzyme that catalyzes transcription and produces RNA (mRNA). Bacteria have a single type of RNA polymerase whereas eukaryotic nuclei have three, RNA polymerase I, RNA polymerase II and RNA polymerase III. It unwinds the DNA for complementary base-pairing in the 5' - 3' direction (downstream). The nucleotides for base-pairing are UTP, ATP, GTP and CTP. The hydrolysis of the nucleotide triphosphate bonds gives energy for transcription to move forward. Unlike DNA replication, transcription does not require RNA primers. However, RNA polymerase make about one mistake per 104 nucleotides, whereas DNA polymerase only makes about one mistake per 107 nucleotides[1]. Although RNA polymerase has a higher error rate than DNA polymerase[2]., RNA polymerase nonetheless have a modest proofreading mechanism which is known as "backtracking" . The RNA-DNA hybrid is able to move in the opposite direction of elongation and remove the wrong base pairs when there is a non-Watson-Crick base pair intorduced[3]. In E.coli RNA polymerase is Mg2+ dependent. It is a multisubunit composed of 5 subunits which can catalyse transcription but cannot recognise or bind to a promoter, it, therefore, requires a sigma factor which allows it to recognise the promotor. The core RNA polymerase plus the sigma factor is called a holoenzyme. Sigma 70 is the main sigma factor, however alternative sigma factors do exist which mediate the binding of RNA polymerase to different classes of promotor. Sigma factor reduces the affinity of RNA polymerase for non-specific DNA, therefore it binds to the correct DNA sequence, called the promoter sequence. The promotor dictates where transcription will start and how efficient transcription will be - a strong promoter sequence will be as close to the consensus sequence as possible.


  1. Alberts, B. et al., 2002. Molecular Biology of the Cell. Fourth Edition. New York: Garland Science.
  2. Alberts B, Johnson A, Lewis J, Martin R, Roberts K, Walter P. Molecular Biology of The Cell 5th Edition. 2008; Chapter 6: How Cells Read the Genome: From DNA to Protein
  3. Berg JM. , Tymoczko JL. , Gatto, Jr. GJ, Stryer L . Biochemistry 8th Edition. 2015; pg 863- 854
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