7. Regulation Flashcards
common regulatory mechanisms in bacteria
regulation of gene expression
-transcription initiation
-transcription elongation
-translation
alter activity of enzymes and proteins
-postrtranslational
three domains of life differ in genome structure and regulatory mechanisms used
regulation of transcription initiation
constitutive genes
inducible genes
repressible gene
constitutive genes
housekeeping genes that are expressed CONTINUOUSLY by the cell
inducible genes
genes that code for inducible enzymes needed only in certain environments (such as beta galactosidase) –> lac operon
repressible genes
enzymes that function in biosynthetic pathways
-generally these enzymes are always present (until turned off) unless the end product in the biosynthetic pathway is available
positive control
regulator protein ACTIVATES the binding of RNA polymerase to DNA
positive control activation
maltose catabolism in E.coli
-maltose activator protein cannot bind to DNA unless it first binds maltose (inducer)
-subsequent binding
ACTIVATOR PROTEINS bind specifically to ACTIVATOR-BINDING SITE (certain DNA sequence that is not called an operator)
*inducer binds to activator protein which binds to activator binding site
*sometimes the activator binding site does not need to be next to the promoter
Positive Control Structure
need activator protein for polymerase to bind
activator binding site
inducer (maltose) binds to maltose activator protein which allows RNA polymerase to bind to the mal Promoter
negative control: repression and induction
the operon
cluster of consecutive genes whose expression is under control of a single operator
-all genes transcribed as single mRNA
-transcription physically blocked when repressor binds to operator
enzyme induction can also be controlled by a
repressor
addition of inducer…
inactivates repressor and transcription can proceed
the repressor’s role is
inhibitory (preventing mRNA synthesis), so it is called negative control
negative control of lac operon
inducible genes
-three structural genes coding for lactose uptake and metabolism
-lac repressor (lacI) binds operator, inhibiting transcription
enzymes normally not produced unless lactose is present
lac operon structure
CAP site is regulatory
lac promoter (RNA polymeraSe binds here)
lac operator
lacZ, lacY, lacA
when the inducer (allolactose) binds to the repressor, RNA polymerase can proceed transcription
diauxic growth
two exponential growth phases if two energy sources available
-better energy source consumed first, growth stops
-after lag, growth resumes with second energy source
*glucose preferentially used , then lactose *
glucose low and lactose present
- polymerase needs to associate
- need to pull off repressor
glucose is low, cAMP is present since the cell is in starvation mode and ATP modified to cAMP
cAMP binds to CAP polymerase and can easily bind
lactose converted to allolactose (binds to repressor) and leaves
regulation of lac operon by lac repressor
regulated by catabolite activator protein CAP
-regulates in response to presence or absence of glucose
-allows for preferential use of glucose
regulation of transcription elongation
-transcription can also be regulated by controlling transcription termination
-this type of regulation, called attenuation, was first demonstrated with trp operon
-more recently riboswitches have been demonstrated to also play a regulatory role
trp Opeorn Attenuation
in addition to transcription initiation control, transcription continuation is also controlled in this operon
attenuation is termination of transcription within the leader region (leader peptide)
occurs through stem-loop structures in the mRNA depending on trp level
low trp level=
transcription continues
riboswitches (sensory RNAs)
specialized form of transcription attenuation
folding of mRNA leader sequences (the riboswitch) determines if transcription will continue/ terminate
folding pattern altered in response to mRNA binding of an effector molecule
riboswitches in gram-positive bacteria function in transcriptional termination
regulation of translation
riboswitches in gram-negative bacteria regulate translation of mRNA
-effector binding elements at 5’ end alters mRNA leader folding pattern
translation initiation can also be controlled by some small RNA molecules
quorum sensing
cell-to-cell communication mediated by small signaling molecules such as N-acyl-homoserine lactone (AHL)
couples cell density and intercellular communication to transcription regulation
plays an essential role in the regulation of genes whose products are needed for the establishment of virulence, symbiosis, biofilm production, and morphological differentiation in a wide range of bacteria
quorum sensing in v. fischeri
high concentrations of AHL produced by increased density of cells diffuse back into the cell, bind to the transcriptional regulator LuxR and activate transcription
LuxR stimulates transcription of the genes for AHL synthase (luxl) and proteins needed for light production
response to autoinducers by v. harveyi
responds to three autoinducers
-maximizes expression of bioluminescence
low cell density
-low autoinducers
-LuxR not made, no bioluminescence
high cell density
-any combination of inducers
-LuxR made, bioluminescence occurs
CRISPR
clustered regulatory interspaced short palindromic repeats
a prokaryotic “immune system” that evades viral destruction and maintains genome stability
