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• Gene Regulation
• The Lactose Operon
• The Tryptophan Operon
• Always on-
Also known as ‘House Keeping Gene’.
a) Normally off-
Also known as ‘Inducible Gene’. Because it
induces or switches on the gene.
b) Normally On-
Also known as ‘Repressible gene’. Because it
represses or switches off the gene.
Activator binds to regulatory site to stimulate
transcription; gene off in the absence of
Repressor binds to regulatory site to stimulate
transcription; Gene is off in absence of
• An operon is a group of genes that are
transcribed at the same time.
• They usually control an important biochemical
• They are only found in prokaryotes.
The lac operon consists of three genes each
involved in processing the sugar lactose
One of them is the gene for the enzyme β-
This enzyme hydrolyses lactose into glucose
• E. coli can use either glucose, which is a
monosaccharide, or lactose, which is a
• However, lactose needs to be hydrolysed
• So the bacterium prefers to use glucose when it
• A repressor protein is continuously synthesised. It
sits on a sequence of DNA just in front of the lac
operon, the Operator site
• The repressor protein blocks the Promoter site
where the RNA polymerase settles before it starts
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• A small amount of a sugar allolactose is formed within the
bacterial cell. This fits onto the repressor protein at another
active site (allosteric site)
• This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the operator
site. RNA polymerase can now reach its promoter site
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• The genes of the trp operon are used to make
the amino acid tryptophan.
• It is turned off when enough tryptophan is in
• Tryptophan is the effector molecule.
• Operon codes for 5 genes in bacteria.
• Five polypeptides combine to make three
• Each enzyme participates in a step to make
• Repressor Protein (trpR) binds to operator
when tryptophan is present.
• Prevents RNA polymerase from transcribing
genes when bound to operator.
• Transcribed as a different gene from trp operon
• Enzymes are required to make tryptophan
• Repressor is NOT bound to operator
• Transcription can proceed
• Tryptophan binds to repressor.
• Repressor can now bind operator to prevent
Operon is set of gene that are coordinately
controlled by a regulatory protein and
transcribed as a single polycistronic message.
This mechanism allows for synchronization of
transcription and translation, a key element in