| The enzymes responsible for the synthesis of RNA, using DNA as a template, are called RNA polymerases. All RNAs are synthesized by these enzymes, in a direction that is 5' to 3' with respect to their internucleotide linkage. This polarity of synthesis dictates that the DNA strand used as a template is read in the 3' to 5' direction (Fig. 31.4). RNA polymerase uses the DNA template strand to synthesize the complementary RNA strand, adding each new nucleotide onto the 3' end of the growing chain. RNA polymerases have the ability to initiate the synthesis of RNA without the benefit of a free 3'-OH on which to build the new RNA strand. This activity distinguishes them from the DNA polymerases (Chapter 30), which require RNA or DNA primers with a free 3'-OH to begin synthesis.
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| RNA polymerases are large multimeric enzymes that transcribe defined segments of DNA into RNA with a high degree of selectivity and specificity
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The RNA polymerases generally consist of two large-molecular-weight subunits and several smaller subunits, all of which are necessary for accurate transcription to occur. In prokaryotic cells, there is only one type of RNA polymerase, which synthesizes all three of the general classes of RNA. In contrast, eukaryotic cells have three RNA polymerases (I, II, and III), which are distinguished from one another by the class of RNA for which they direct the synthesis. The function of each of the eukaryotic RNA polymerases was determined in part by using a transcription inhibitor, α-amanitin - a toxic compound found in some mushrooms.
- RNA polymerase I synthesizes the rRNAs;
- RNA polymerase II synthesizes mRNA. It is extremely sensitive to inhibition by α-amanitin;
- RNA polymerase III synthesizes the small RNAs, including the tRNAs.
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| Figure 31.4 Transcription. Transcription involves the synthesis of an RNA by RNA polymerase using DNA as a template. The RNA polymerase holoenzyme uses one strand of DNA to direct the synthesis of an RNA molecule that is complementary to this strand. |
| An otherwise healthy young man presents himself to the emergency room with severe nausea and diarrhea. After his medical history has been taken, he explains that his symptoms came on rather suddenly, about 2-3 hours after he had eaten dinner. The physician suspects some form of food poisoning, and asks the patient to recall everything he has eaten over the past 24 hours. The only suspicious food mentioned were mushrooms that the patient ate for dinner. The mushrooms become prime candidates for the cause of this patient's symptoms when he further relates that they were picked up on a recent hike through the woods. What is the biochemical basis for suspecting that the mushrooms are the cause of this man's illness? |
| Comment. It is likely that the patient mistakenly picked a member of the family Amanita phalloides and ingested them at dinner. The toxin, α-amanitin, binds preferentially to RNA polymerase II and inhibits its function; if a large quantity of the mushrooms had been ingested, even RNA polymerase III could be inhibited. The first cells that encounter the toxin are those of the digestive tract, leading to acute gastrointestinal distress: cells that are incapable of synthesizing new mRNAs and tRNAs would die, causing the diarrhea and nausea that the patient complained of when first examined. |
| The best-studied RNA polymerase is that of Eschericha coli. The core portion of this bacterial polymerase is a tetramer containing two α subunits and a β and β' subunit, which interact to form what is called the core polymerase. This polymerase is capable of synthesizing RNA, but does so in a nonspecific fashion. It gains specificity and is able to bind and initiate transcription at true initiation sites on DNA when a fifth subunit, the σ-factor, joins the complex. E. coli contains a number of different σ-factors that, when joined with the core polymerase, impart specificity for the transcription of certain classes of genes.
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