RNA Polymerase

Introduction to RNA polymerase

RNA can be synthesized by RNA Polymerases The discovery of DNA polymerase and its reliance on a DNA template lacked a hunt for an enzyme that synthesizes RNA complementary to a DNA strand.  From 1960, four study teams had independently discovered a receptor in cellular extracts, which may produce an RNA polymer from ribonucleoside 5 -triphosphates.

After work on the purified Escherichia coli RNA polymerase assisted in specifying the essential attributes of transcription. DNA-dependent RNA polymerase wants, together with a DNA template, all four ribonucleoside 5-triphosphates (ATP, GTP, UTP, and CTP) as precursors of these nucleotide components of RNA, along with Mg2. The protein also binds one Zn2.  The chemistry and mechanics of RNA synthesis closely resemble those used by DNA polymerases. RNA polymerase elongates an RNA strand with the addition of ribonucleotide units into the 3 -hydroxyl end, building RNA from the 5 n3 direction.  The 3 -hydroxyl group acts as a nucleophile, attacking the phosphate of the incoming ribonucleoside triphosphate and eliminating pyrophosphate.

Mode of Action

RNA polymerase demands DNA for action and is active when bound into some double-stranded DNA.  As mentioned above, just one of both DNA strands serves as a template.  The template DNA strand is replicated in the 3n5 management (antiparallel to the new RNA strand), as in DNA replication.  Each nucleotide in the recently formed RNA is chosen by Watson-Crick base-pairing interactions.; U residues are inserted into the RNA to match with A residue in the DNA template, G residues are placed into the RNA to match C residues, etc.   Base-pair geometry may play a role in the base choice.

 Unlike DNA polymerase, RNA polymerase doesn't need a primer to initiate regeneration.  Initiation happens when RNA polymerase binds at specific DNA sequences known as promoters (explained below).  The 5 -triphosphate set of the first residue in a nascent (newly formed) RNA molecule isn't cleaved to eliminate PPi, but instead stays intact during the transcription procedure.  During the elongation stage of transcription, the growing end of this new RNA strand base-pairs temporarily using the DNA template to create a tiny hybrid RNA-DNA dual helix, estimated to be eight bp long. The RNA inside this hybrid "peels off" shortly after its creation, and also, the DNA duplex re-forms to Permit RNA polymerase to synthesize an RNA strand complementary to one of these DNA strands, and the DNA duplex should loosen within a brief distance, forming a "transcription bubble". During transcription, in E-coli, the RNA polymerase generally holds about 17 bp unwound.  The 8bp RNA-DNA hybrid befalls within this unwound location. The E-coli RNA polymerase proceeds the elongation of the RNA transcript at a speed of 50 to 90 nucleotides/s.

Since DNA is a helix, the motion of a transcription bubble demands substantial strand rotation of these nucleic acid molecules. As a result, a traveling RNA polymerase creates waves of positive supercoils and negative supercoils in front and behind of transcription bubble.   It had been detected both in in-vitro and in-vivo (in bacteria).  With in the cell , the topological problems caused by transcription are relieved via the action of topoisomerases.   Both complementary DNA strands have different purposes in transcription. The strand that acts as a template for the synthesis of an RNA transcript is known as the template strand. The complementary strand of the template possesses the same number of base pairs as the RNA transcribed from the template DNA strand possesses, along with U residues in the RNA transcript rather than T residues in the DNA.  The coding strand for a specific gene might be found in either strand of a particular chromosome.  The regulatory sequences that control transcription are by convention characterized by the strings in the coding strand.

A remarkable similarity has been found between the RNA polymerases seen in prokaryotes, eukaryotes, archaea and even some viruses. It indicates that they may have evolved from one common ancestor. Prokaryotic RNA polymerase is composed of four subunits, named as a sigma-factor that dissociates from the receptor enzyme complex following transcription initiation. Prokaryotes employ precisely the exact same RNAPolymerase to catalyze the polymerization of RNA nucleotides. There are five different RNA polymerases in eukaryotes.

 RNA polymerase one in eukaryotes is a workhorse, making almost fifty percent of the RNA transcribed from the template DNA in the cell. It only polymerizes ribosomal RNA, which is a large part of ribosomes, the molecular machines involved in protein synthesis. RNA Polymerase II is widely researched since it is found in the transcription of mRNA precursors. Additionally, it catalyzes the synthesis of micro   RNAs and small nuclear  RNAs.  RNA polymerase  III transcribes tRNA, a few ribosomal RNAs along with some other small RNAs and plays a  significant role in the normal operation of the cell. RNA polymerases IV and V are found solely in plants, and are essential for the synthesis of small interfering RNA and also heterochromatin within the cell nucleus.    

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