Lecture 48: The lac operon (REVISIT) Flashcards
Tuesday 18th February 2025
Is the lac operon used for regulating genes?
Yes
What is an operon?
- group of genes under the control of the same promoter
- common in prokaryotes
- many genes can be regulated together.
Polycistronic mRNA
encodes more than one protein, only in prokaryotes
Are all genes protein encoding?
No
Operon doesn’t always have to encode proteins, can encode other thing as well.
How many rRNAs does e coli have?
3
Housekeeping genes
Genes that are required to be active at all times, i.e involved in replication and transcription. Necessary for cell survival
Housekeeping genes
Consittutively expressed
Why are other genes, not housekeeping, regulated?
Because very energy costly process
Genes are switched on and off
Genes are switched on and off
Lactose
- disaccharide
- galactose and glucose (disaccharide)
- ## found in milk
Glucose is favoured by ecol, but will use lactose if it haves to
Diauxic growth
- 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
azi
Azide (poison for bacteria)
What proteins do the lac genes encode?
- lac Y: encodes beta-galactoside -permease; allows lactose to enter the cell
- lac Z: encodes beta-ga;actodsidase; 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
Lac operon off
- 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 operatior. (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
Allolactose
- 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.
-
How can allolactose frmation be castalysed by beta-galactosidase (lac Z) if operon is repressed?
- 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
rna pol
- lac i comes off promoter, allowing rna polymerase to bind
- translation initiated, 3 proteins made
Why is there a lag? Why weren’t the lactose genes already turned on?
- Glucose actively suppresses the lac operon as well
- This is known as CAP, whcih is needed to enhance trancription
- 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
Glucose inhibits cAMP production
Glucose inhibits cAMP production So wehn we hae glucose in the system we dont have cAMP.
Once glucose is used up, we can start making cAMP and transcription can start
Once glucose is used up, we can start making cAMP and transcription can start
Lac operon and biotechnology
- 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
Why is the lac promoter a good choice?
- Strong promote,r makes lots of mrRNA
- Don’t wasnt our gene to be constitutively expressed, due to energy cost
- However, gowing on lactose is incovenient. glucose is more convenient
glucose more convenient….
many mutations in operons htat don’t require glucose for activationl
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.
Variants of lac promoter are insensitive to glucose, so grown on glucose.
Variants of lac promoter are insensitive to glucose, so grown on glucose.
