Transcription in eukaryotes happens in the nuclear compartment. Unlike the prokaryotes, mRNA translation in eukaryotes happens almost simultaneously before the completion of RNA synthesis. The newly synthesised mRNA is readily transported to the cytoplasm.
Therefore, one end of the mRNA is under construction, and the other end that passes to the cytoplasm will be translated immediately, as soon as the ribosomes are attached to it. The details of the transcription mechanism in eukaryotes are explained here.
Enzymes Involved in Eukaryotic Transcription
However, similar to the prokaryotes, RNA polymerase is the catalyst for eukaryotic RNA synthesis. Here, it is more complex, comprising RNA pol I, RNA pol II, and RNA pol III. These are DNA-dependent and carry transcribed genes for all types of RNA.
RNA pol I transcribes rRNA
RNA pol II transcribes mRBA genes
RNA pol III can transcribe tRNA and other types of small RNAs as well.
RNA Polymerase I
Transcribes genes for only rRNA and produces 70% of total cellular RNA>
It requires two supportive factors called UBF (Upstream Binding Factor), which are polypeptides that bind to SLIGC-rich core promoters.
It is located in the nucleolus.
RNA Polymerase II
Located in the nucleoplasm.
Synthesizes hnRNA or heterogenous nuclear RNA, which is a precursor of mRNA>
Gets initiated by a low concentration of ⍺-amination.
Consists of 10 subunits.
The large subunit has CTP (carboxyl-terminal domain), characterised by a repeated sequence of 7 amino acids.
RNA Polymerase III
Located in the nucleoplasm.
Has minor enzyme activity.
Synthesizes 10% of cellular RNA, specifically tRNA and small RNA.
Transcription Mechanism in Eukaryotes
Eukaryotes have several promoter genes that are organised 30 nucleotides ahead of the transcription site.
The promoter TATA box is rich in A=T, which is located 25 base pairs away.
This promoter is identified by eukaryotic RNA polymerase II.
CAAT box and GC box are located at -40 and 110 regions and are recognised by RNA pol II.
Eukaryotic transcription involves several transcription factors that bind to many cis-acting elements.
Such factors bind to the DNA upstream sequence that influences transcription initiation.
These factors may recognise RNA polymerase or can also be incorporated into the initiation complex when other proteins are present.
The basal apparatus comes in contact either directly or indirectly.
The transcription unit consists of a gene.
The termination of synthesis by RNA pol I and pol III at a specific sequence in a particular distance from the coding region.
But the termination mechanism of RNA pol I is yet to be clear.
Initiation
As the initiation complex is formed at the promoter region, the initiation process starts.
The first step is binding of factor TF II D, followed by TF II A joining the complex.
At the same time. TF II F does not bind directly to this complex but brings the RNA polymerase to the site.
Apart from these, TF II E and TF II H also join the complex.
TF II H, which has ATPase and helicase activity, controls the movement of RNA synthesis. It continues as the RNA pol II moves on from the initiation site.
After initiation, the transcription factors are released from the DNA.
Pic Credit: Cooper GM. Sunderland (MA): Sinauer Associates; 2000.
Elongation
The TF I IH uses an ATP molecule to unwind the DNA helix to facilitate transcription.
The RNA polymerase II with a non-phosphorylated CTD initiates the transcription.
TF II D will also dissociate the RNA polymerase II by phosphorylating it and changing its conformation.
The phosphorylation occurs at the C-terminal tail known as the C-terminal domain (CTD).
The RNA polymerase II will now move along the DNA template and elongate the transcription.
Elongation occurs from 5’ to 3’.
The newly formed RNA molecule is called the primary transcript.
The RNA polymerase II in eukaryotes does not terminate the transcription process at any specific site.
In eukaryotes, it stops the transcription process at varying distances anywhere on downstream of the gene.
The cleaving is done at an internal site, releasing the transcript for further processing.
The 5’ exonuclease digests the remainder of the transcript.
It will also help the RNA polymerase II disengage from the DNA template, thus completing the termination process.
Conclusion
The DNA transcription enables gene expression and occurs as a preparation for protein translation. The newly transcribed RNA will be subject to further processing.
Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Eukaryotic RNA Polymerases and General Transcription Factors. Available from: https://www.ncbi.nlm.nih.gov/books/NBK9935/