Key Knowledge:
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Genes can be categorised as being either structural or regulatory:
- Structural genes code for proteins that contribute to the overall form or function of a cell or organism
- Regulatory genes code for proteins that control levels of gene expression by altering transcription rates (i.e. switch genes on or off)
Gene regulation is mediated by a group of proteins called transcription factors (also known as regulatory proteins)
Transcription Factors
Transcription factors either mediate or impede the binding of RNA polymerase to the promoter (and hence control transcription rates)
- Transcription factors that prevent transcription by impeding RNA polymerase action are called repressors
- Factors that promote transcription by increasing the affinity of RNA polymerase for the promoter are called activators
In prokaryotes, transcription factors bind to a region of DNA called the operator (proximal to the promoter) in order to moderate RNA polymerase binding
- In eukaryotes, transcription factors bind to more distant regions (silencer or enhancer sites) to regulate levels of overall transcription
The expression of transcription factors can be mediated by cell signals (e.g. hormones) that may be triggered by internal or external cues
- E.g. Humans may produce different levels of melanin (skin pigment) according to their overall level of sun exposure (i.e. sun tanning)
The trp Operon
The trp operon provides a simplified example of the basic elements of gene regulation in a prokaryotic cell
- An operon is a cluster of genes under the control of a single promoter
The trp operon codes for a number of enzymes that coordinate to promote the synthesis of tryptophan (a non-essential amino acid)
- The expression of the operon is controlled by a transcription factor called the trp repressor (produced by an upstream regulatory gene)
- The trp repressor binds to an operator site (adjacent to promoter) and inhibits expression by preventing the action of RNA polymerase
Tryptophan binds to the repressor and triggering a conformational change that activates the protein
- When tryptophan levels are high, the trp repressor is active and the operon is switched off (preventing over-expression of tryptophan)
- When tryptophan levels are low, the trp repressor is inactive and the operon is switched on (so more tryptophan will be synthesised)
Attenuation
Attenuation is an alternative method of reducing the expression of the trp operon in prokaryotic cells
- It relies on the capacity for prokaryotes to undertake transcription and translation at the same time (as the DNA is cytosolic)
- Whereas the trp repressor system blocks the initiation of transcription, attenuation blocks the completion of transcription
Located between the operator and trp genes is a leader sequence that encodes a short polypeptide containing two Trp residues
- When transcribed, this leader sequence contains several self-complementary sections that are capable of forming hairpin loops
- One such loop is called the attenuator and functions as a site of transcriptional termination when it is formed (transcription halts)
- Alternatively, an earlier loop called the anti-terminator may form, which prevents attenuator formation (so transcription continues)
The presence of tryptophan determines whether the attenuator or anti-terminator is formed within the leader sequence
- If tryptophan is in low supply, the ribosome will pause at the two Trp codons within the leader sequence
- This pause allows time for the anti-terminator to form on the mRNA transcript, so transcription will continue (operon is expressed)
- If tryptophan is in high supply, the ribosome will not need to pause at the two Trp codons within the leader sequence
- This means the anti-terminator is unable to form and the attenuator will form instead (transcription halted ; operon not expressed)
