Gene Expression Can Be Regulated at SEVERAL DIFFERENT Levels
- 1) Transcription
- 2) RNA Processing
- 3) RNA Stability
- 4) Translation
- 5) Post-Translation
Most genes are regulated at the level of TRANSCRIPTION
- Whether a gene is copied into RNA
- How much transcription occurs from a given gene
- Occurs promarily through binding of proteins to DNA to medulate RNA pol.
Most Gene Products are Only Needed at CERTAIN times/cells
Induction: turning ON a gene in response to something in the environment.
Repression: turning OFF a gene in response to something in the environment.
Transcription Can Be:
INCREASED if an activator protein binds RNA Polymerase.
DECREASED if a repressor protein blocks RNA polymerase from binding.
encodes proteins used in metabolism or for 'structure'
encode products that control the expression of other genes
Change the ability of the a regulatory protein to bind DNA
E. Coli Metabolism
- Prefers GLUCOSE as an energy source
- When NO glucose is present, can use other carbs (lactose, sucrose, galactose)
- Genes for metabolizing these sugars are turned off until needed
- Some genes are essential to all cells and are expressed constitutively.
- Constitutively = a few genes, particularly those that encode essential essential cellular functions, are expressed continually.
ex: tRNA, rRNA, ribosomal proteins, RNA and DNA polymerases
Lactose Metabolism in E.Coli
- Francois Jacob & Jacques Monod 1961
- Nobel 1965
Lactose Metabolizing Genes are Grouped Together Into One Operon
- Z,Y, & A are structural genes!
- LacZ encodes B-Galactosidase
- LacY encodes Permease
- LacA encodes Transacetylase
Negative Control of the Lac Operon
Repressor protein is bound to the operator preventing RNA polymerase from binding and no transcription of Z, Y, A = Transcription OFF
Negative Induction control of the Lac Operon
Inducer molecule binds the repressor and prevents it from binding the operator. RNA polymerase can now bind and transcribe Z, Y, & A genes.
What is the inducer molecule?
The inducer, although it appears to be lactose, it is actually allolactose. Upstream of lacP is a regulator gene, LacI, which has its own promoter (PI). The lacI gene is transcribed into a short mRNA that is translated into a repressor. Each repressor consists of 4 identical polypeptides and has 2 binding sites.
A group of bacterial structural genes that are transcribed together (along with their promoter and additional sequences that control transcription). The operon regulateds the expression of the structural genes by controlling transcription, which, in bacteria, is usually the most important level of gene regulation.
How can you make allolactose if the lac operon is off?!
- The lac operon is not 100% off.
- Sometimes the repressor falls off!
- A little B-galactosidase and permease are always present.
When Lactose is present:
- 1) Small amounts of B-gal metabolize lactose.
- 2) The allolactose that is formed binds to the repressor.
- 3) The repressor + allolactose complex cannot bind the operator.
- 4) RNA pol binds and lots of B-gal and permease are made (1000x increase).
Glucose is the #1 preferred energy source for E.coli!
- When glucose is PRESENT:
- - lac operon is not needed
- - induction of the lac operon is actively repressed
Catabolite Repression Regulatory Molecules:
CAP: catabolite activator protein
- cAMP: cyclic AMP
- - the effector/inducer
- - made from ATP
CAP bind cAMP to form CAP/cAMP complex.
High glucose, no lactose
Glucose present (cAMP low); no lactose; no lac mRNA
- cAMP is too low to form CAP/cAMP complex
- lac repressor is bound to the operator
High glucose, + lactose
Glucose present (cAMP low); lactose present
cAMP is too low to form CAP/cAMP complex; lac repressor is bound by the inducer; nothing is bound to the operator
Result: Very low level of lac mRNA
No Glucose, + lactose
No glucose present (cAMP high); lactose present
- cAMP is high, so it forms CAP/cAMP complex
- CAP/cAMP complex binds the promoter and enhances binding of RNA polymerase
- lac repressor is bound by the inducer
Result: very high levels of lac mRNA