regulation of gene expression Flashcards
microbe response
microbes respond immediately to enviro changes
- chemotaxis: movement towards fuel or light source
- facultative aerobe: switch from O2 to alternate e-acceptor
- temperature change - increase or decrease in unsaturated FA in membrane
how pathogens signal pathogenesis
express virulence factors to establish infection
- express genes to establish symbiont-host relationship
gene expression in prokaryotes
start @ promotor
stop @ terminator
RNA polymerase creates single mRNA transcript form stuff in the middle
polycistronic mRNA
one transcript encodes multiple polypeptides; ORF separated by stop codons + intercistronic regions of varying length
what are polycistronic genes
high gene density on DNA. some genes read forward, others in reverse. tighlty regulated
RNa polymerase, promoter + sigma factor recognition sites
need RNA pol+ sigma factor together to read off rna.
diff sigma factors interact w rna pol to get diff sequence from same/similar mrna strand
what is probnow box?
10 nucleotides form start. from -10 to -35 sigma + holoenzyme start trasncribing from
process of transcription
RNA pol + sigma factor come together at promoter region.
- sigma recognizes promotoer + initiation site
- trxn starts, sigma released. RNA chain grows to termination
- chain stops
- release polymerase and RNA
= once RNApol moves down, another guy can hop on at same time
transcription termination
DNA contains inverted repeats
- complementary base-pair stem-loop inhibits continuation of trxn.
physical interference with movement of RNA pol
coupled trxn and translation
both in cytoplasm. as RNa strand formed, ribosomes hop on and synthesize protein.
polysome:
translation of single mRNA by several ribosomes
= rapid turn on/off. exponential synthesis
4 ways of regulating activtiy
- constitutive expression - no control
- metabolic or posttranslational control of enzyme activity
- regulation of enzyme synthesis at translation
- regulation of enzyme synthesis at transcription
regulation of enzymes
posttranslational
- direct control of enzyme activtiy
- rapid
allosteric regulation
regulation of transcription
- operon
- structural genes
- operator
- promoter
- regulator
operon: genetic element that controls synthesis of several proteins in prokaryotes
structural genes that encode polypeptides
operator: starts/halts txn by binding protien factors
p: bind RNapol for txn
R: encodes single protein (green circle) which binds to operator to control gene expression
negative transcriptional regulation
block mRNA synthesis
- enzyme overly abundant
- activity no longer necessary or detrimental to cell
- involves “repressor protein” to bind DNA and block RNApol
positive transcriptional regulation
stimulate mRNA synthesis
- more enzyme needed in cell
- activator protein bind to DNA and promote transcription
what are constitutive genes?
housekeeping
- no regulation
repressible genes
expression stopped when product not needed
- level of enzyme decreases in presence of repressor and co-repressor
inducible genes
expressed only when need arises
b-galactosidase induced when glucose is gone, lactose is present
- catalyze lactose hydrolysis to galactose and glucose
- level of enzyme rises in presence of repressor and inducer
negative control - enzyme reduction ex: arginine added to e.coli on media.
enzyme for argninine shut down to conserve energy. arginie corepressor to prevent own synthesis.
what is corepressor
repressor cannot bind operator alone. corepressor needed to bind to repressor to bind to operator. once bound no more synthesis
negaative control: enzyme induction
e.coli growing on substrate other than lactose.
lactose added = enzymes induced + transcribed.
- de novo enzyme synthesis
how lactose presence = enzyme induction
repressor bound at operator of lac operon.
lactose = inducer: binds to repressor causing its removal, txn allowed
positive control
- activator protein promotes binding of RNA polymerase promoter
- non-continuous operons involved
- inducer molecule required for activator to bind DNA
- maltose is inducer
catabolite repression
expression of genes to metabolize non-preferred sugar are repressed due to lack of catabolite (cyclic AMP). want to use preferred substrate first. non-preferred second.
what is diauxic growth?
glucose metabolized first
- gene expression for lactose metabolism turns on B-galactosidase
- lag during switch in gene expression and de novo enzyme synthesis. biphasic growth curve.
transcription of lac operon onlly occurs when?
glucose levels are low AND lactose is present
Catabolite activator protein + lac operon?
CAP - bind to site upstream ofpromoter in lac operon to stimulate transcription (+regulator)
CAP bound to catabolite, cAMP, to be active.
glucose transport inhibits cAMP synthesis and lactose transport
catabolite repression lifted by adding caMP to media
lac operon under both positive and negative control
CAP binds to cAMP = cap-cAMP dimer.
bind to lac operon increases binding of RNA polymrase at promoter
- txn of lac genes begins
-lactose required as inducer to remove repressor protein bound at operator (negative control of lac operon)
summary of lac operon regulation
- Lactose, no glucose. repressor inactive, CAP–cAMP stimulates txn
- galactose + glucose. allolactose take off repressor. cAMP levels too low to activate CAP no trxn
- no lactose, no glucose. both activator and repressor are active. no trxn
- glucose, no lactose. no caMP and repressor block txn
CAP as global regulator
regulates several operons under control of different catabolites
- arabinose, galactose, maltose, + other carbon sources.
- each operon under control of negative regulators too.
- some features differ for each operon, binding by CAP-cAMP is same
what is regulon
multiple operons responding to common regulatory protein - CAP operon. all the stuff that CAP moderates
good things about global regulation
multiple operons allow bacteria to quickly adjust to changing enviro
what happens when alternate sigma factors are produced?
sigma factor = nutrient deprivation, creates spore.
global effects
= cell differentiation
= tolerance to abrupt temp changes
= increase in osmolarity
e. coli regulons
O2 both +/-, [caMP], temp, -NH3
oxygen radical
damaged DNA
two-component regulatory systems and phosphorelay
two component signals link external events to regulation of gene expression
- transfer of phosphoryl group from one protein to another
signal transduction
- sensory kinase
- response regulator
sK: respond to enviro signal “effector molecules”. become phosphorylated
RR: phosphorylated by sensor kinase. becomes activator of gene expression. dephosphorylated by phosphatase enzyme
OmpF/OmpC porin proteins
f - wider diameter than c= more nutrients through.
C protects against bile salts
f - used when osmotic pressure is low.
c when osmotic pressure is high
regulation by two component signal transduction, envZ for OmpF repression
EnvZ - sensor kinase - sense increase in osmolarity. autophosphorylates and passes phosphoryl group to OmpR, the response regulator
- ompR-P can bind to DNA to repress ompF and activate ompC expression
quorum sensing
regulation in response to cell density. regulated by autoinducer.
-> diffusible signaling, molecule in membrane: acetyl homoserine lactone (AHL)
found in g- bacteria, species specific, and synthesized in cell and diffused out.
triggers cascade - binds to regulator (enhancer) quorum binds to operator to recruit polymerase. lux increases. shift from none to lots of transcription when there are a certain number of cells
lux operon
cells glow due to luciferase enzyme.
= bioluminescent bacteria.
encodes AHL synthase.
luxR(activator) binds AHL (inducer) and positively regulates expression of lux genes = bioluminescence
best studied bacterium for bioluminescence
vibrio fischeri. luminescence takes place only at hgih cell density when quorum sensing genes are induced.
bacterial communications
cross-talk, “eavesdrop”, sensing, signaling molecules.
= stop cross talk may be antimicrobial drug source? no signal that enough cells to trigger infection = no onset of infection
antisense RNA
transcribed from opposite DNA strand of mRNA.
- base pair with mRNA to prevent translation,(+) control or degrate RNA.
regulate synthesis from synthesized RNA
translational control: riboswitches
g(-) bacteria
= regulate translation of mRNA
- absence of ligand, mRNA forms stem-loop and access to binding sequences (shine-dalgarno). in presence of ligand, diff interaction = diff stem-loop no access to ORF (shine-dalgarno)
