Chapter 24- Regulation of Gene Expression Flashcards
Genes expression can be regulated at different levels. What are examples of ways to regulate gene expression?
- Transcriptional level (copies of mRNA)
- RNA post-transcriptional modification
- Stability of mRNA (balance synthesis and degradation)
- Translational level (rate)
- Post-translational modification level (glycosylation, proteolytic cleavage, phosphorylation)
How can mRNA- transcriptional level be regulated?
number of mRNA molecules/unit time
translation of mRNA
number of copies of a gene
control of mRNA regulation in prokaryotes
controlled by regulating initiation of transcription
gene expression fluctuate in response to changing environment
Lifetime of prokaryotic mRNA
few minutes
Enzymes of prokaryotic pathway are encoding in
polycistronic mRNA, one mRNA translation produces all of the enzymes
control of mRNA regulation in eukaryotes
controlled by regulating transcription initiation and post-transcriptional modification stages
Transcription of eukaryotic genes are
monocistronic; one mRNA for one protein
Lifetime of eukaryotic mRNA
hours or days
Key regulators in prokaryotic gene expression are
the first substrate or end product
Negative Regulation:
regulatory protein present gene off
gene has active promoter, inhibitor or repressor, keeps transcription turned “OFF”, anti-inhibitor, inducer turn the system “ON”
Positive Regulation:
regulatory protein present gene on
gene has inactive promoter, effector or activator molecule activate promoter
lacZ gene encodes
β-galactosidase
lacY gene encodes
lactose permease
lacA gene encodes
thiogalactoside transacetylase
What is the lac operon?
a set of of adjacent genes transcribed as one polycistronic mRNA + the adjacent regulatory signals
In the absence of Lactose what happens to lac operon regulation?
LacI repressor binds to lacO site and repress lacZYA transcription
In the presence of Lactose what happens to lac operon regulation?
allolactose binds to lacI repressor causing a conformational change in the protein that prevents binding to lacO: Transcription of lacZYA genes activates
Mutant studies proved that in lacI:
- β-galactosidase expression is inducible
- LacI is required for repression of β-galactosidase
- LacI can compliment in trans
Mutant studies of LacO proved that
- operator is required for repression
2. operator region function in cis
When glucose and lactose present in media what is the preferred source of carbon?
glucose
If glucose and lactose present what happens to β-galactosidase expression?
no induction
If glucose is high what happens to cAMP and β-galactosidase expression?
cAMP is low
no β-galactosidase expression
If glucose is low what happens to cAMP and β-galactosidase expression?
cAMP is high
β-galactosidase is expressed
If there are mutations in adenylate cyclase (cya) gene and cry gene (encoding CAP or CRP) what happens to β-galactosidase expression?
decreased β-galactosidase expression
Glucose high, cAMP low then
low cAMP-CAP -> no binding to the promoter -> no activation of β−gal expression
If Glucose low, cAMP high then
high cAMP-CAP -> binding to the promoter -> activation of β−gal expression
What is purpose of Trp operon?
synthesize tryptophan
How is Trp operon regulated?
excess trp represses trp expression
What levels of regulation is involved with trp operon?
Transcriptional initiation
Feedback inhibition
Transcriptional attenuation
What is Transcriptional initiation?
Trp repressor protein (trpR) binds to operator in trp operon -> repress transcription
What is Feedback inhibition?
[Trp] is high, trp binds to the first enzyme in the pathway -> inactivate the enzyme, temporary shut-off of trp biosynthesis
What is Transcriptional attenuation?
Termination hairpin
Anti-termination hairpin
What is temporal regulation of gene expression?
Selective and sequential transcription of particular sets of genes
What is phage T7 major types of transcription events?
Early transcription- host sigma
Late transcription- phage RNA polymerase
Bacteriophage T4 has groups of genes?
Immediate-early genes (IE)
Delayed-early genes (DE)
Late genes (L)
Immediate-early genes (IE):
use host RNAP, some IE-gene products degrade host DNA, others bind to a subunit of host RNAP, making it no longer recognize host cell promoters
Delayed-early genes (DE):
modified host RNAP recognizes DE genes, T4 genome replication by DE products, one of DE products is a novel sigma factor for the late phage genes
Late genes (L):
modified RNAP no longer express DE genes but transcribes L genes, synthesizing new capsid proteins, tails, and tail fibers, and assembly proteins, Finally, phage T4 lysozymes are activated
How is the T4 genome replicated
as a rolling circle, producing extremely long concatemers
What unusual base does T4 contain in its DNA?
5-hydroxymethylcytosine (HMC)
What is bacteriophage lambda?
temperate phage
What lifecycle does bacteriophage lamb have?
lytic and lysogenic
