gene expression Flashcards
regulation of gene expression in bacteria
- a bacterium adapts depending on the environment
- genes that are not required are not expressed unless environmental conditions change (regulation is very tight)
levels of gene regulation
- alteration of structure
- transcription
- mRNA processing
- RNA stability
- Translation
-post-translational modification
constitutively expressed genes
genes that specify products that are essential components of living cells
- need all the time and are always turned on
inducible and repressible genes
- needed for cell growth or only under certain environmental conditions
- regulatory mechanisms allow the synthesis of these gene products only when they are needed
ligand
binds to a repressor activator and tells it these are the conditions under which you should bind to the DNA
Negative control: operon types
Repressible: default mode is on
inducible: default mode is off
Negative control: operon types
Repressible: default mode is on
inducible: default mode is off
positive regulation in bacteria
- involves an activator which when bound allows transcription of mRNA
positive inducible genes
- usually off (activator not bound)
- in absence of ligand binding to activator, gene is turned off
- if ligand binds to activator, activator will bind to promotor and transcription will happen
positive repressible genes
- usually off (activator bound)
- when ligand binds to activator it leaves the promoter and transcription no longer happens
negative regulation in bacteria
- repressor is involved which when bound prevents transcription of mRNA
negative repressible genes
- usually on (repressor not bound)
- when ligand binds to repressor, repressor binds and strops transcription
negative inducible
- usually off (repressor bound)
- when ligand binds to repressor it leaves and transcription can happen
Positive control: CAMP and CRP
- low glucose causes buildup of cyclic AMP (cAMP)
- camp binds to regulatory protein CRP
- this activates the protein which binds to promoter of the operon
- allows RNA polymerase to bind and begin transcription (turns on)
- as glucose levels rise cAMP levels lower, decreasing rate of transcription
induction of genes for lactose utilization
- for utilization of lactose gene expression is induced when lactose is present and glucose is absent
- alters the rate of lactose-metabolizing enzyme synthesis
- enzymes involved in catabolic pathway are often inducible (off)
repression of genes for tryptophan biosynthesis
- genes are expressed in the absence of tryptophan and turned off when tryptophan is available
- enzymes involved in anabolic pathways are often repressible (on)
operon
a series of related genes that are expressed coordinately through the use of a single promoter and operator
- most structural genes with related functions in bacteria are organized into operons for efficiency of gene expression
negative inducible operons
- usually off
- the regulator protein is active and bound to the operator and prevents transcription of the structural genes
- then the inducer is present it binds to the regulator making it unable to bind to the operator and transcription takes place
inducer
ligand that helps to express an operon
- may bind to repressor to take it off or bind to activator to put it on
negative repressible operons
- usually on
- regulator protein is an inactive repressor, unable to bind to operator, transcription takes place
- levels of corepressor build up which bind to regulator protein making it active and able to bind to operator, preventing transcription
positive control of gene expression
the regulatory protein “activator” binds to DNA and interacts with RNA polymerase to assist the efficiency or transcription of structural genes
The lac operon of E.coli
- makes lactase to break down lactose
- genes are transcribed only when lactose is present and glucose is absent
- consists of regulatory repressor “I” gene (contains its own promoter), a second promoter “P”, a regulatory operator “O”, and 3 structural genes (Z,Y,A)
operator
- a piece of sequence that sits after the promoter and binds to repressor
lac operon system
- lactose premease (Y gene): lactose transporter (in membrane)
- B-galactosidase (Z gene): main purpose is to turn lactose into glucose and galactose, low background level converts lactose (allolactose - lac operon inducer)
- lac I region: encodes lac repressor, diffusible tetramer that represses in the absence of all lactose
- lac O region: the lac operator, regulatory DNA sequence that works in cis to control lac operon, binding site
lac operator
- overlaps the initiation start site
- when bound by lac repressor RNA poly can’t access promoter
when is the lac operon ON or OFF
ON: in the presence of allolactose
- lac repressor binds inducer and is inactive
- induction results in transcription of lac Z, Y, and A genes
OFF: in the absence of the inducer allolactose
- the lac repressor binds to the lac operator and prevents RNA poly from transcribing Z, Y and A
what happens when there is a mutation in the lac operator
- genes are always on
constitutive lac mutants
- caused by mutations in the I gene (I-) or the operator (Oc)
- Oc mutations act only in cis
- wild type Z+, Y+ and A+ are dominant (from functional enzymes) and I+ is dominant
lacl superrepressors
- some lac mutants were never inactivated by allolactose
- in these mutants the allolactose binding site in the lac repressor gene is altered
- lacl repressor is always bound to the operator, preventing transcription of genes
- in a diploid, Lacls is dominant over lacl+
Is the lac operon always induced in the presence of allolactose
- no, not if glucose is also present
- glucose prevents lac operon induction- negative regulation
- aka catabolite repression
how does glucose regulate the lac operon
- indirectly through cAMP levels
- when glucose is high, cAMP is low
- when cAMP is high (low glucose), it binds to CAP which permits binding to a special region in the lac promoter - required for lac operon transcription
positive control of the lac operon (low glucose)
1) cAMP/CAP binding
2) RNA poly binding
- cAMP/CAP orients RNA poly and associated sigma factor to start transcription
