Lecture 9 - regulation of gene expression Flashcards
At what points can bacteria control gen expression?
- control of transcription initiation
- mRNA stability
- control of translation
- post translational control (protein modification and degredation)
When is regulation most effective and efficient?
Earlier in the expression pathway
Transcriptional regulatory proteins
DNA binding proteins that increase/decrease expression of proteins by sensing changes and altering gene expression
Where are regulatory proteins encoded?
- either adjacent to the sequence they regulate…
- or far away if it regulates multiple genes (regulon)
Major regulatory strategies
- repression: transcription inhibited by repressor proteins that block RNA pol from binding promoter
- activation: transcription requires an activator protein to begin
How do repressor proteins usually work?
Cause steric inhibition that prevents RNA pol from accessing promoter
How do activator proteins usually work?
Binds adjacent to the promoter to stabilize the area for RNA pol binding
How are regulator strategies modulated?
Small molecules increase/decrease affinity of regulatory proteins for their operator sequences
Two examples of transcriptional repression mechanisms
- Repressor binds DNA and represses target gene; inducer causes repressor to release
- Repressor-corepressor complex binds DNA and represses target gene; corepressor removal causes repressor to release
LacZ
aka beta-galactosidase; breaks bond between galactose and glucose
LacY
aka lactose permease; allows import of lactose into cytoplasm
Steps of transcription induction of lac operon
- LacI repressor binds operator and prevents RNA pol from binding to lac promoter
- Lactose converted to allolactose by low levels of beta-galactosidase (LacZ)
- Allolactose binds LacI repressor and reduces its affinity for operator
Example of transcriptional activation mechanism
Activator binds ligand (inducer) –> complex binds regulatory sequence and activates target gene –> removing ligand stops transcription
Activation of lac operon by cAMP-CRP is an example of…
transcription induction via inducer ligand (cAMP)
Steps of lac operon activation via cAMP-CRP
- cAMP is made from ATP in low-energy conditions
- cAMP forms complex with cAMP receptor protein (CRP)
- cAMP-CRP complex binds promoter and interacts with RNA pol to increase transcription
- increased transcription of proteins for alternative metabolic pathways
Derepression of the trp operon
- when trp levels exceed cellular needs, excess tryptophan acts as co-repressor for TrpR
- trp operon transcription is repressed –> decreased synthesis of tryptophan
- when trp becomes limiting, is dissociates from TrpR –> TrpR falls off operator and trp transcription resumes
Why is repression of transcription important?
Cell wants to limit energy usage from unnecessary protein production
Is transcription repression complete?
No. Every so often, repressors will dissociate and produce small amounts of the transcript.
How are transcription and translation coupled?
Polysome structure: Multiple RNA pols make multiple transcripts that are being translated by multiple ribosomes per transcript
How do multiple ribosomes translate one mRNA strand?
Once the first ribosome moves down the mRNA and re-exposes the ribosome binding site and start codon, another small unit can bind.
What happens to transcription of the trp operon when trp is limiting?
- When trp is limiting, ribosomes pause at a trp-trp codon sequence in the leader peptide sequence because there aren’t as many tRNA molecules available with trp attached.
- Ribosome pausing causes mRNA strand to adopt a hairpin antiterminator structure; no interference with RNA pol activity
- Transcription is allowed to continue
What happens to transcription of the trp operon when trp is plentiful?
- When trp is plentiful, ribosomes move quickly through the trp-trp codon sequence in the leader peptide sequence because there are many available tRNA molecules with trp
- Since ribosome is not paused to obstruct the strand, crosslinking occurs with a neighboring portion of the mRNA molecule –> terminator conformation is made with a stem-loop structure right behind RNA pol
- Stem-loop structure signals transcription termination
Relationship between trp levels and trp operon transcription
- high trp –> low trp operon transcription
- low trp –> hig h trp operon transcription
Where is the trp-trp sequence located on the trp operon?
Between the leader region and the region encoding trp operon products
- if no transcription terminator, whole operon is transcribed
- if transcription terminator is made, operon is not transcribed
