Regulation of transcription in bacteria I Flashcards
Why not all genes are expressed at once? (prokayotes)
– Different genes express
different environments
• presence of a food substrate (lactose)
– Don’t waste energy expressing genes which
are of no use in a particular environment?>, MNB
What determines the different types of cells?
– All cells retain entire hereditary potential of the
organism
– BUT differentiate into different types (brain vs
muscle) so need to express different factors
Eukaryotic cell have off ground state why?
1- whole bunch of proteins beside RNA polymerase are needed to turn the gene on
2- Genes are not readily available for expression, Because they are wrapped around nucleosome in a chromosome
The structure of a eukaryotic chromosome
In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure
DNA wrapped around histone—> form nucleosome–> few nucleosome form chromatin
How gene expression is controlled in prokaryotic cells in presence of sugar?
Bacteria sense and respond to particular stimuli
often very rapidly (within minutes)
• Example
– Bacteria can utilise many different sugars for growth
– Only express the genes necessary for metabolism of
the sugar when it is present
• Must be able to
– Sense when the sugar is present
– Up-regulate necessary genes in response
– Down-regulate necessary genes when sugar is
absent
two levels of regulation in prokaryotes
Regulation at the level of transcription
– Most common form of regulation
– Transcription of DNA into mRNA
– Can regulate how much mRNA is made from a gene
– If no mRNA then final protein can’t be expressed
• Regulation at the level of translation
– Translation is the synthesis of protein from an mRNA
message
– Regulation of how much active protein is made from an mRNA transcript
what is negative regulation?
Negative regulation (repressor)
– A regulatory protein is used to stop transcription of a
gene (turns promoter OFF)
– When genes need to be turned on in response to a
stimulus the negative regulator must be inactivated
What is positive regulation?
• Positive regulation (activator)
– A regulatory protein is required before the gene is
efficiently transcribed (turn promoter ON)
– When genes need to be turned on in response to a
stimulus the positive activator must be activated
regulation of transcription mostly occurs at …..
the initiation point
what is the role of the promotor sequence?
Promoter sequences determine where RNA
polymerase starts transcription
– For eg. TTGACA-17bp-TATAAT
All genes have slightly different promoters
One RNA polymerase – thousands of genes.
Are they all transcribed simultaneously?
No– many alternative sigma factors recognising alternative promoter sequences.
different sigma factors are produced for different promotor
The contraction of sigma factor produced can vary based on the external signal
sigma factor 32
recognizes the promotor on the genes encoding for chaperons
level of sigma factor 32 increases when temperature increases , leading to sigma factor binding to the promotor increasing the expression of chaperons
what determine which gene is transcribed ?
the bacterial cell has many alternative sigma
factors, all competing for binding to the core RNA
polymerase.
What genes are transcribed is determined by how
abundant this or that sigma factor is at this particular
moment
when the promotor is off ?
– A promoter is “off” when RNA polymerase can’t bind and begin transcription
– The repressor usually binds over the promoter sequence so RNA pol can’t bind to the DNA
where does the repressor binds to?
The place where the repressor binds is called the
“operator site”
how can repressor stop the transcription?
and what happens in presence of inducer?
1-Repressor binds to operator and blocks transcription
Repressor binds to inducer and no longer binds to operator
What is an co-repressor?
in some instances a co-repressor molecule may be
required to allow the repressor to bind.
– Repressor plus co-repressor becomes active for binding
How the inducer block the activity of the repressor?
- The repressor binds to the operator site and
stops transcription - When the inducer is present it binds to the
repressor and the repressor falls off the DNA - RNA polymerase can now access the promoter and perform transcription
what is activator proteins what is it’s role?
An “activator” protein is required to bind to
the DNA to turn a promoter on
– The promoter is only active when the activator
binds
• The activator can only bind DNA in the
presence of an inducer molecule
– The inducer interacts with the activator
allowing it to bind to the DNA
• In the absence of an inducer molecule
– the activator does not bind to DNA so the
gene is off
Activation is induced by
inducer
1. The activator must bind to the promoter
to allow transcription to begin
2. The activator can only bind to the DNA in the
presence of the inducer
What are allosteric proteins?
Allosteric proteins are proteins that exist in two
conformational forms (different shapes)
Transcriptional repressors and activators are allosteric
proteins
These proteins change conformation when they
bind the inducer
– Change in conformation alters ability to bind DNA
how many binding site does the allosteric protein have?
• These allosteric proteins have at least two
different binding sites
– One for binding the inducer
– One for binding DNA
How lac Operon is regulated?
Lactose (lac) operon of E. coli
– Controls metobolism of lactose
• Negative regulation
– By the lactose repressor (product of lacI)
– Inducers are low MW b-galactosides (allo-lactose)
• Positive regulation
– by CAP protein
– Inducer is cAMP (which responds to glucose)
bacterial operons
group of gene that are transcribed together as they have a common promotor
they usually have similar function
what happens when glucose is present in the bacteria?
Glucose is the most efficient carbon source for
bacteria
– So if glucose is present it is metabolised preferentially
to other sugars including lactose
– lac operon will not be expressed when glucose is
present
positive regulation in lac operon
Mediated by levels of cAMP and the positive
regulator CAP (catabolite activator protein)
– Called “catabolite repression
Catabolite Repression of the lac Operon
Glucose levels regulate cAMP levels
high glucose level inactivate the adenylate cyclase which converts ATP to cAMP
cAMP-CAP complex activates transcription by binding to the promotor
what is the role of CAP?
CAP activates transcription after binding cAMP
– CAP is an allosteric protein
– Induces DNA bending
– Bent DNA allows RNA polymerase to access the
promoter more efficiently
• CAP only activates transcription efficiently in the
absence of Lac repressor (consider the situation when there is no glucose and no lactose present)
So high levels of lac operon expression when
– Glucose is absent and
– Lactose is present
What happens when Glucose is present (cAMP low) and no lactose
Repressor binds
CAP doesn’t bind
NO expression of lac mRNA
What happens when Glucose is present (cAMP low)
and lactose present
Repressor doesn’t bind
CAP doesn’t bind
Little expression of lac mRNA
What happens when Glucose is absent (cAMP high)
and lactose present
Repressor doesn’t bind
CAP binds
High expression of lac mRNA
