Promoter and TATA box in transcription initiation
The promoter is the DNA region at the start of a gene where transcription factors (proteins that attach to DNA and switch genes on) gather to start transcription.
Remember
A mutation that weakens transcription factor binding at a promoter, such as at the TATA box (a short DNA sequence found in many promoters), reduces transcription, so less mRNA is made and less protein usually follows, without changing the protein's amino acid sequence.
Memory aid
The promoter is the switch that allows transcription to begin, whereas an enhancer is more like a dimmer.
Use the switch for the promoter and the dimmer for an enhancer to keep their jobs apart.
Transcription is the copying of a gene's DNA into messenger RNA (mRNA). For a gene to be transcribed, RNA polymerase (the enzyme that builds the RNA) has to find where it starts. This is done by the promoter, a DNA region lying upstream of (before) the coding sequence, the part that is read to make the protein.
Transcription factors attach to the promoter and recruit (bring in) RNA polymerase. In many eukaryotic genes (genes of cells with a nucleus) the promoter contains the TATA box, a short run of DNA with the consensus (typical) sequence TATAAA, sitting about 25 to 35 bases before the point where transcription starts. A TATA-binding protein, part of the complex TFIID, recognises it. The other factors then fall into place and recruit RNA polymerase II.
The TATA box is rich in A-T base pairs, which are held together by weaker bonds, so the DNA unwinds easily there and the polymerase can open the double helix. Not every gene has a TATA box, so remember it as one of the sequences that transcription factors can recognise, not the only one.
Enhancers (DNA sequences that can lie some distance from the gene) are different. They raise the rate of transcription, but transcription can proceed without them. The promoter is the switch that allows transcription to begin, whereas an enhancer is more like a dimmer.
A mutation in the promoter changes how much RNA is made, and leaves the protein's sequence as it was. If the factors bind poorly, RNA polymerase is recruited less often, so less mRNA is made. Because the coding sequence is unchanged, any mRNA that is made still codes for a normal protein: the problem is quantity, not quality.
Key facts
- Promoter
- the DNA region at the start of a gene where transcription factors gather and RNA polymerase is recruited.
- TATA box position
- in many eukaryotic genes it sits about 25 to 35 bases before the point where transcription starts.
- Recognised by
- a TATA-binding protein in TFIID, after which the other factors fall into place.
- Not every gene
- the TATA box is present in some genes and is one of the sequences that can be recognised by transcription factors.
- Enhancers
- raise the rate of transcription from a distance but are not necessary for transcription to proceed.
- Promoter mutation that weakens factor binding
- less mRNA and usually less protein, with the amino acid sequence unchanged.
Exam traps
- An enhancer is not the sequence sitting just upstream of the start site that helps position RNA polymerase, because it raises the rate of transcription from a distance and transcription can proceed without it.
- The polyadenylation signal (the sequence that marks where a tail of A nucleotides is added to the transcript) does not start transcription, because it acts on the RNA after it has been made.
- A disrupted TATA box does not make transcription factors bind more tightly, because it weakens binding of the factors that recognise it.
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