Lecture 18 control of gene expression Flashcards
Why do prokaryotes only make proteins when they are needed?
To conserve energy and resources
How do prokaryotic cells shut off supply of unneeded proteins?
- Downregulate the transcription of mRNA
- Hydrolyze mRNA after it is made
- Prevent translation of the mRNA at the ribosome
- Hydrolyze the protein after it is made
- Inhibit the function of the protein
What must a prokaryotic cell do in order to shut off supply of unneeded proteins?
- Respond to the environmental signals
- Must be efficient
Why is transcriptional regulation preferred?
It is the most efficient because it conserves energy
What is the preferred energy source for E.coli?
Glucose
What is lactose?
B-Galactoside: galactose-B linked to glucose
What three proteins are needed for E.coli to take up and metabolize lactose?
- B-galactoside permease
- B-galactosidase
- B-galactoside transacetylase
How are the three genes for lactose uptake situated in relation to each other?
The three genes for lactose uptake and metabolism
are adjacent on the chromosome
How are the three genes for lactose uptake/metabolism linked?
The three genes share one promoter and are
transcribed together
What is an operon?
Operon: unit comprising two or more structural genes,
one promoter and one operator.
What is the lac operon?
The lac operon contains the genes for lactose metabolism
What are structural genes?
Structural genes encode protein or RNA products which
are not regulatory factors.
What is an operator?
Operator: stretch of DNA which binds regulatory proteins.
What is a repressor protein?
Repressor protein binds to the operator and
prevents transcription.
What is the name of the operon containing the genes for the three lactose-metabolizing E.coli genes?
The lac operon
What are the two binding sites on the repressor protein?
One for the operator and one for the inducers
What are the inducers for the lac operon?
Molecules of lactose
How does binding with the inducer effect the repressor?
changes the shape of the repressor protein by allosteric modification, prevents the repressor binding to the operon
What happens when the concentration of lactose drops?
The inducer molecules (lactose) separate from the repressor, the repressor returns to its original shape and binds to the operator.
Why does translation stop after the transcription of the lac operon stops?
Because the mRNA that is already present breaks down quickly
What is the name of the type of system in which the inducer regulates binding of the repressor?
An inducible system
What encodes for repressor genes?
Regulatory genes
What is the regulatory gene that codes for the repressor of the lac operon called?
i (inducibility) gene
Where is i in relation to the operon it regulates?
Close, however, some regulatory genes are distant from their operons
What is the promoter of the i gene called?
pi (the i is subscript)
How is the amount of repressor protein controlled?
The promoter’s gene, pi, does not bind to RNA polymerase effectively, only enough to synthesize 10 repressor molecules per generation
What is the name given to the type of gene of the repressor for the lac operon?
Constitutive- it is made at a constant rate (there is no operator between pi and i gene
No environmental control
Operator- repressor control ______ transcription in the lac operon.
Induces
Operator-repressor control ________ transcription in the trp operon
Represses
Why does E.coli switch of the enzymes for the amino acid tryptophan?
When it is present in ample concentrations, it is advantageous to switch off this gene
What does it mean for a protein to be repressible?
It can be turned off in response to biochemical cues
How does a repressor protein shut off its operon in repressible systems?
It first binds to a corepressor
Where does a corepressor come from?
It is either the end product itself (trytophan) or an analog of it
What do inducible systems generally control?
Catabolic pathways
What do repressible systems generally control?
Anabolic pathways
How else can protein synthesis be controlled?
By increasing promoter efficiency
What is the series of steps in the lac operon for the promotor to bind RNA polymerase?
Regulatory protein CRP binds to nucleoside adenosine 3’,5’-cyclic monophosphate (cyclic AMP, cAMP).
CRP-cAMP complex binds to DNA upstream of the promotor
What is the result of CRP-cAMP complex binding upstream of the DNA promotor?
More efficient binding of RNA polymerase to the promoter and thus elevated levels of transcription of structural genes
What happens when there is high glucose and lactose present?
High glucose leads to low levels of cAMP, so CRP does not undergo conformational change and therefore RNA polymerase is not asissted in binding to promoter
What is it called when a preferred energy source represses other catabolic pathways?
Catabolite repression
What is the inducible lac operon and repressible trp operon examples of?
negative control of transcription.
What is negative control of transcription?
a repressor prevents transcription
What is positive control of transcription?
an activator increases transcription
What is an example of positive control?
The cAMP-CRP control of the lac operon
What are the boundaries between introns and exons called?
Consensus sequneces
What are consensus sequences?
Short stretches of DNA that appear with little variation in many genes
How is gene expression regulation different to DNA replication regulation?
In DNA replication, it is all or nothing,
In gene expression, it is highly selective
What are the two types of regulation that are responsible for differences in proteins among cell types?
Transcriptional regulation
Posttranscriptional regulation
How can transcriptional regulation and posttranscriptional regulation be distinguished?
By examining the actual mRNA sequences made within the nucleus of each cell type
What are housekeeping genes?
Those that encode for proteins involved in basic metabolic processes that occur in every living cell
How is the structure of genes in eukaryotes and prokaryotes different?
Prokaryotes have functionally related genes grouped into operons
Eukaryotes have solitary genes
How are several eukaryotic genes expressed at once?
Common control elements in each of the genes allow all of the genes to respond to the same signal
How are DNA polymerases different in prokaryotes and eukaryotes?
Prokaryotes have just one
Eukaryotes have 3
Which RNA polymerase catalyzes the transcribes protein-coding genes in eukaryotes?
RNA polymerase II
How can eukaryotes regulate the rate of their transciption?
Additional sequences in their genes
What are the two sequences of a prokaryotic promoter?
Recognition sequence- recognised by RNA polymerase
TATA box- where DNA begins to dentature so the template strands can be exposed.
What must assemble on the chromosome before eukaryotic RNA polymerase can bind to the promoter?
Transcription factors
What is the first step in eukaryotic transcription factors assembling on a chromosome?
The protein TFIID binds to the TATA box
What happens when TFIID binds to the TATA box?
It changes the shape of TFIID and the DNA, presenting a surface that attracts other transcription factors to form a transcription complex.
What are specific promoters important in?
Differentiation (specialization of cells during development)
Where are regulator sequences found in eukaryotic cells?
Upstream of the promoter
What binds to the regulator sequence?
Regulator proteins (such as beta-globin gene) bind to these sequences- these bind to adjacent transcription complexes and activate it
What does TFIIH do?
denatures DNA
What does TFIIB do?
helps identify the initiation site
What does TFIIF do?
helps recruit RNA polymerase
What does TFIIE do?
helps stabilise denatured DNA
What is an enhancer sequence?
A regulatory sequence which binds activator proteins to activate transcription or increases rate of transcription.
What is a silencer sequence?
regulatory sequence which binds factors that repress transcription.
What is the purpose of bending the DNA?
To bring distant regulatory sequences, and their associated proteins, into contact
