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Eukaryotic Transcription
Eukaryotic Transcription
Dr.V.Malathi
Eukaryotic Transcription
Eukaryotic transcription is carried out in the nucleus of the cell . Eukaryotes also employ three different polymerases that each transcribe the various subset of genes. The process take place in three stages as
- Initiation
- Elongation
- Termination.
Eukaryotes require proteins, called transcription factors, to first bind to the promoter region which then help to recruit the appropriate polymerase.
Eukaryotic RNA Polymerases
The three different RNA polymerases in eukaryotes, each comprise of 10 subunits or more and require a distinct set of transcription factors to bring the RNA polymerase to the DNA template.
RNA polymerase I is located in the nucleolus. Transcribe the genes for Ribosomal RNA ( r RNA)- transcribes the 28S, 18S, and 5.8S rRNA genes. . These are structural RNAs has they have a cellular role but are translated in to protein.. The rRNAs are components of the ribosome and are essential to the process of translation. RNA polymerase I synthesizes most of the rRNAs.
RNA polymerase II is located in the nucleus. Synthesizes all pre-mRNAs and small nucleolar RNAs (snoRNAs) involved in rRNA processing and small nuclear RNAs (snRNAs) involved in mRNA processing, except for U6 snRNA. The eukaryotic pre-mRNAs undergo extensive processing after transcription but before translation.
RNA polymerase III is also located in the nucleus. This polymerase transcribes a variety of structural RNAs including tRNA, 5S rRNA, U6 snRNA, and the 7S RNA associated with the signal recognition particle (SRP). The tRNAs have a critical role serve as the adaptor molecules between the mRNA template and the growing polypeptide chain. Small nuclear RNAs have a variety of functions, including “splicing” pre-mRNAs and regulating transcription factors.
| RNA Polymerase | Cellular Compartment | Product of Transcription | α-Amanitin Sensitivity |
| I | Nucleolus | All rRNAs except 5S rRNA | Insensitive |
| II | Nucleus | All protein-coding nuclear pre-mRNAs | Extremely sensitive |
| III | Nucleus | 5S rRNA, tRNAs, and small nuclear RNAs |
Moderately sensitive |
Each of the types of RNA polymerase recognizes a different promoter sequence and requires different transcription factors.
The RNA polymerase transcribes one strand, the antisense (-) strand, of the DNA template. RNA synthesis does not require a primer and occurs in the 5’ → 3’ direction .
The RNA polymerase catalyzes a nucleophilic attack by the 3′-OH of the growing RNA chain on the alpha-phosphorus atom of an incoming ribonucleoside 5′-triphosphate.
All three of the nuclear RNA polymerases are complex enzyme . Each consisting of 8 to 14 different subunit . Although they recognize different promoters and transcribe distinct classes of genes, they share several common features.
The two largest subunits of all three eukaryotic RNA polymerases are related to the β and β′subunits of the single E. coli RNA polymerase.
In addition, all three eukaryotic polymerases have common five subunits .