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The enzyme RNA polymerase or RNAP is a nucleotidyltransferase that polymerises ribonucleotides in accordance with the information present in DNA. RNA polymerase enzymes are essential and are found in all organisms.

RNAP accomplishes de novo synthesis. It is able to do this because specific interactions with the initiating nucleotide hold RNAP rigidly in place, facilitating chemical attack on the incoming nucleotide. Such specific interactions explain why RNAP prefers to start transcripts with ATP (followed by GTP, UTP, and then CTP). In contrast to DNA polymerase, RNAP includes a helicase activity, therefore no separate enzyme is needed to unwind DNA.

RNAP was discovered independently by Sam Weiss and Jerard Hurwitz in 1960.

 

RNA polymerase in prokaryotes

In prokaryotes, the same enzyme catalyzes the sythesis of all three types of RNA: mRNA, rRNA and tRNA.

RNAP is a relatively large molecule. The core enzyme has 5 subunits (~400 kDa):

• α₂: the two α subunits assemble the enzyme and recognize regulatory factors.
• β: this has the polymerase activity (catalyzes the synthesis of RNA).
• β': binds to DNA (nonspecifically).
• ω: function not known clearly.However it has been observed to offer a protective/chaperone function to the β' subunit in M.smegmatis.

In order to bind promoter-specific regions, the core enzyme requires another subunit, sigma (σ). The sigma factor greatly reduces the affinity of RNAP for nonspecific DNA while increasing specificity for certain promoter regions, depending on the sigma factor. The complete holoenzyme therefor has 6 subunits: α₂ββ'σω (~480 kDa). The structure RNAP exhibits a groove with a length of 55 Å and a diameter of 25 Å. This groove fits well the 20 Å double strand of DNA. The 55 Å length can accept 16 nucleotides.

 

RNA polymerase in eukaryotes

Eukaryotes have several types of RNAP:

• RNA polymerase I synthesizes a pre-rRNA 45S, which matures into 28 S, 18S and 5,8S rRNA's
• RNA polymerase II synthesizes precursors of mRNA's and most snRNA. This is the most studied type, and transcription factors are required for its binding to its promoters.
• RNA polymerase III synthesizes tRNA's, rRNA 5S and other small RNA's found in the nucleus and cytosol.
• Other RNA polymerase types in mitochondria and chloroplasts.

 

Isolation

RNA polymerase can be isolated in the following ways:

• By a phosphocellulose column (PC).
• By glycerol gradient centrifugation (GG).
• By a DNA column (A).
• PC+A