Transcription in eukaryotes

Transcription in eukaryotes

How a gene in the nucleus becomes a strand of pre-mRNA, and how the three RNA polymerases share the work.

Transcription in the nucleus
RNA polymerase II sits on the DNA double helix, holding a short stretch open while it moves along it. A strand of pre-mRNA leaves the polymerase and trails back towards the promoter, where TFIID stays bound to the TATA box. The pre-mRNA’s 5′ end carries a cap. Below, the nuclear envelope is drawn as two lipid bilayers.

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Pol II has left the promoter and moves to the right, reading the template strand. Simplified, not to scale.

At a glance

  1. RNA polymerase II transcribes protein-coding genes into pre-mRNA. RNA polymerase I makes most rRNA (28S, 18S, 5.8S) and RNA polymerase III makes tRNA and 5S rRNA.
  2. Pol II reads the template strand 3′→5′ and builds RNA 5′→3′, so the RNA matches the coding strand, with U in place of T.
  3. Pol II cannot find a promoter on its own. General transcription factors assemble first: TFIID binds the TATA box, where there is one, about 25 to 30 base pairs upstream of the start site, through its TATA-binding protein (TBP).
  4. Pre-mRNA is processed while it is made: a 5′ cap, splicing out of introns, and a 3′ poly(A) tail.

Initiation

Transcription starts at the promoter. TFIID binds the TATA box through TBP, then TFIIB, TFIIF with Pol II, TFIIE and TFIIH join, in the order seen in the test tube, to form the preinitiation complex. TFIIH opens the DNA with its helicase activity and phosphorylates serine 5 in the C-terminal domain (CTD) of Pol II, which releases the polymerase from the promoter.

Elongation

Inside the transcription bubble, a short stretch of the DNA is held open. Pol II adds ribonucleotides to the free 3′ end of the growing chain, pairing each one with the template strand. Behind it the helix closes again, and the new pre-mRNA leaves through an exit channel.

Processing as it goes

Once the RNA is about 25 nucleotides long, its 5′ end gets a 7-methylguanosine cap. The cap protects the RNA from degradation and helps the ribosome find it later. The spliceosome starts removing introns while transcription continues, and the 3′ end is cleaved a little downstream of the AAUAAA signal and given a poly(A) tail of about 200 adenines.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1Which enzyme makes most mRNA in human cells?

2In which direction is RNA made?

3Two days after eating wild mushrooms, a patient develops acute liver failure. Which process did the toxin block?