Lecture 48: The lac operon (REVISIT) Flashcards
Tuesday 18th February 2025 (48 cards)
1
Q
Is the lac operon used for regulating genes?
A
Yes
2
Q
Describe an operon?
A
- A group of genes under the control of the same promoter
- Common in prokaryotes
- Many genes can be regulated together.
3
Q
Describe Polycistronic mRNA
A
Encodes more than one protein, only in prokaryotes
4
Q
Are all genes protein encoding?
A
No
5
Q
Operon doesn’t always have to encode proteins, can encode other thing as well.
A
Operon doesn’t always have to encode proteins, can encode other thing as well.
6
Q
How many rRNAs does e coli have?
A
3
7
Q
What are housekeeping genes
A
- Genes that are required to be active at all times, i.e involved in replication and transcription.
- Necessary for cell survival
8
Q
Is it true that housekeeping genes are Consituitively expressed?
A
Yes
9
Q
Why are other genes, not housekeeping, regulated?
A
Because it’s a very energy costly process
10
Q
Genes are switched on and off
A
Genes are switched on and off
11
Q
Describe lactose
A
- Disaccharide made up of galactose and glucose
(disaccharide) - Found in milk
12
Q
Glucose is favoured by ecol, but will use lactose if it has to
A
Glucose is favoured by ecol, but will use lactose if it has to
13
Q
Describe Diauxic growth
A
- Growth of e coli with glucose and lactose
- Has 2 growth phases
- First is glucose being used up
- 2nd is lactose being used up
- Lag phase is where glucose has run out and so genes for breaking down lactose will be turned on
- Carbon sources are used consecutively, not at the same time
14
Q
What is azi the code for?
A
Azide (poison for bacteria)
15
Q
What proteins do the lac genes encode?
A
- lac Y: encodes beta-galactoside -permease; this allows lactose to enter the cell
- lac Z: encodes beta-ga;actodsidase; this cleaves lactose
- lac A: encodes galactoside acetyl-transferase; enzyme that transfers acetyl group on to galactasides; we don’t know why, maybe detoxification; doesn’t break down lactose
16
Q
Lac operon off
A
- lac i gene that has its own promoter and is transcribed from another promoter, Promoter, operator, genes (z,y,a)
- lac i gene makes a lac i protein that binds to the operator. This represses the operator. (repressor protein)
- When lac i is bound to the operator, the operon is turned off, so RNA polymerase can’t come and bind to the promoter
- Normal state of the operon is off because e coli won’t waste time utilising a rare sugar when it’s not around
17
Q
Describe Allolactose
A
- An inducer turns gene on an disables repressor, allolactose.
- Like lactose but has bind between carbon 1 and 6 instead of carbon 1 and 4 (so isomer of lactose).
- Lac z cleaves lactose and also turns lactose into allolactose.
18
Q
How can allolactose frmation be castalysed by beta-galactosidase (lac Z) if operon is repressed?
A
- Even in the prepresses state, there is a small amount of transrciortion
- lac z can still kind of bind to the operator, allowing some action from dna poylmerase
19
Q
rna pol
A
- lac i comes off promoter, allowing rna polymerase to bind
- translation initiated, 3 proteins made
20
Q
Why is there a lag? Why weren’t the lactose genes already turned on?
A
- Glucose actively suppresses the lac operon as well
- This is known as CAP, which is needed to enhance transcription
- CAP has 2 diff conformations.
- one confirmation can bind to promoter and makes it easier for RNA pol to bind
- When allolactose is bound by repressor, operon is turned on and cap bins as well
21
Q
Glucose inhibits cAMP production
A
Glucose inhibits cAMP production so when we have glucose in the system, we don’t have cAMP.
22
Q
Once glucose is used up, we can start making cAMP and transcription can start
A
Once glucose is used up, we can start making cAMP and transcription can start
23
Q
Lac operon and biotechnology
A
- We can add our own genes to the operon to make it make proteins for use
- Used to make recombinant proteins in e coli
- We can produce protein for biotech i.e insuilin
- We can produce protein to study them i.e structure and function
- Works becasue genetic code is universal. can insert eukaryotic gene into operon
24
Q
Why is the lac promoter a good choice?
A
- Strong promoter, makes lots of mrRNA
- Don’t want our gene to be constitutively expressed, due to energy cost
- However, growing on lactose is inconvenient. glucose is more convenient
25
glucose more convenient....
many mutations in operons htat don't require glucose for activation
26
lactose inconvenient....
- Can give a ocmpund called IPTG. (lactose analogue)
- Glactoside part is enough for e coli to think that there's lactose in the system, e coli cant tell the difference.
- IPTG can't be metabolised, so concentration will stay the same.
27
Variants of lac promoter are insensitive to glucose, so grown on glucose.
Variants of lac promoter are insensitive to glucose, so grown on glucose.
28
Describe the ribosomal operon in E.coli
- Produces 16S, 23S, and 5S rRNA from a single promoter.
- Not all genes code for proteins — some, like rRNAs, have structural roles.
29
What is meant by the fact that housekeeping genes are constitutively expressed?
They are always turned on
30
Why are only some genes constitutively expressed (always on)?
To conserve energy, as RNA and DNA synthesis is costly
31
In minimal media, with only glucose, E.coli must make all of its amino acids. Why are the genes for making amino acids turned off in rich medium?
Because the amino acids are provided in the rich media, so turning off the amino acid making genes saves energy.
32
What is the lac operon used to do?
The lac operon is used by E. coli to metabolize lactose, a rare sugar.
33
What 3 genes does the lac operon contain?
lacZ, lacY, lacA
34
What is the default state of the lac operon?
Off
35
How is the lac operon kept off in the default state?
- LacI repressor protein binds to the operator (Op) and blocks transcription.
- LacI repressor protein binds to the operator (Op) and blocks transcription.
36
How is the lac operon turned on?
- When lactose is present, a small amount is converted to allolactose (an inducer).
- Allolactose binds LacI, causing it to release from the operator.
- This de-represses the operon — RNA polymerase can now transcribe lacZYA.
- Fun Fact: Even though LacZ is repressed, a tiny amount is still made ("leaky" expression) — enough to make allolactose!
37
Describe how glucose represses the lac operon via catabolite repression
- It inhibits adenylate cyclase, lowering cAMP levels.
- No cAMP → CAP (Catabolite Activator Protein) can't bind DNA.
- Without CAP-cAMP, transcription is weak even if LacI is removed.
38
What happens when glucose is no longer repressing the lac operon?
- cAMP levels rise
- cAMP binds CAP → CAP binds upstream of the lac promoter
- RNA polymerase is recruited efficiently → lac genes are strongly transcribed
39
Why is IPTG preferred to lactose in biotechnology?
Because it is not broken down like lactose + no catabolite repression with modern lac promoter variants
40
What is meant by the fact that the lac operon is inducible?
It is turned on by lactose, via allolactose
41
What is the key thing about operons?
They allow coordinated expression of multiple genes from a single promoter.
42
What does catabolite expression ensure?
Catabolite repression ensures E. coli uses glucose first.
43
What is needed for the full activation of the lac operon?
CAP-cAMP is needed for full activation of the lac operon.
44
What is the lac promoter widely used in biotechnology for?
Widely used for controlling gene expression
45
Where does RNA polymerase bind to?
The promoter
46
Where do repressors bind?
To the operator
47
What happens when alloctolase binds to the lac repressor?
- The lac repressor can no loner bind to the operator site
- This allows RNA polymerase to bind to the promoter and transcribe the lactose genes that will allow lactose to be metabolised
48
