Signal Transduction and Regulatory Cascades Flashcards
How can free living bacteria sense their environment?
Temperature, nutrients, osmolarity, pressure, salinity etc.
What are some types of environmental stimuli?
Nutrients- carbon sources, nitrogen sources, oxygen, light, minerals, trace elements.
Danger- Osmotic changes, pH changes, toxins, phage attack, extreme heat, extreme cold.
Minerals – phosphorus, sulfate, iron. Sense these and swim towards them (if possible), but at least make the most of them when they are available.
Nitrogen- bacteria can sense different forms (ammonia, nitrate etc.) Some can fix nitrogen themselves.
What are the different speeds of response?
Fast response- Immediate (within milliseconds or seconds)- transduction of external signals required.
Slow response- Considered (minutes or hours)- changes in gene expression, proteome profile, or cell morphology, will take time.
Always transient- Whether fast or slow, adaptations of bacterial populations to the environment is transient. Even the most extreme differentiation responses (sporulation) are actually reversible in the long run (in terms of the population as a whole).
Describe the general structure of sensory reception systems.
Stimulus to sensors/receptors.
Transducer/trasmitter.
Reciever.
Regulator.
(these three make up the adaptor).
Global network- genes, modulons, regulators.
Response- metabolism, differentiation, behaviour.
Bacteria monitor their surroundings by an impressive array of extracellular and intracellular sensors. Sensors convert a stimulus from the environment into a signal. Signals are communicated by trasnducers to regulators, and regulators cause the cellular response through modulation of an enzymatic activity or gene regulation.
Always modular- based on electronic circuitry.
What are two component systems?
Most common way bacteria sense stuff in their environment- eg. E.coli have about 30 of these.
Sensory transmitter integrated into inner membrane.
Cell needs to know about stuff that gets past the outer membrane into the periplasm.
2 hydrophobic helices in inner membrane, sensory domain on top (in periplasm).
Response regulator in the cytoplasm.
Describe the sensors in a two component system?
Sensory domain.
Histidine kinase “transmitter” domain, and histidine phosphohistidine on helix 1.
Catalytic domain on helix 2.
A kinase uses ATP to phosphorylate another molecule (usually protein or lipid in biology).
(Phosphohistidine- found in prokaryotes, not much in eukaryotes).
Describe the response regulators in a two component system?
“Helix-turn-helix” domain.
Aspartate phosphoaspartate.
“Reciever” domain and DNA binding domain. Level of phosphorylation of ‘receiver’ domain controls level of DNA binding activity.
(a phosphatase de-phosphorylates another molecule).
Typical hydrogen-bond 2-6 kcal/mol.
Conserved aspartate residue.
Describe the phospho-relay mechanism of two component systems.
Sensory domain detects an extracellular signal and activates the transmitter domain.
Transmitter domain is an auto-kinase and uses ATP to phosphorylate a histidine.
Reciever domain is a phosphatase and takes Pi from transmitter domain for one of its own aspartates.
Output (eg.gene expression).
When a stimulus is removed, the sensory domain returns to a resting state.
Transmitter domain can now adopt a phosphatase activity itself.
Reciever domain is relieved of its Pi by the transmitter domain.
Downstream effect is turned off.
Are two component systems all the same?
Not all the same.
Sense different things.
Front- CheA to CheB or CheY- tell cell whether to swim towards nutrient or not.
What is Vibrio cholerae?
Gram negative curved rod.
Two circular chromosomes (I = 2.9 Mbp; II = 1.07 Mbp).
Found in seawater, estuaries, freshwater, waste water.
Causative agent of cholera, but can form harmless symbiotic relationships too.
Cholera toxin induces adenylate cyclase activity to increase cAMP levels in the host cell.
Activates PKA -> opens chloride channels and other channels leading to ‘secretory diahorea’.
What is chitin?
Long polymer of N acetyl glucosamine (GlcNAc).
10^11 tonnes produced annually in marine waters alone, but only traces of chitin on ocean floor.
Chitinases are secreted by microbes.
GlcNAc is the molecular equivalent of the full English breakfast.
Chitin- 2nd most abundant polymer.
Bacteria break down this very well, despite its toughness.
What is the product of chitinases on chitin?
Chitinases usually produce chitobiose (GlcNAc)2 from chitin.
Describe the Vibrio cholerae chitinolytic cascade.
Takes advantage of any carbon/nitrogen source in water.
Dedicated 2 component system to detect chitobiose.
Porins in the outer membrane- bate barrel protein specific for chitobiose.
Chitobiose proteins in periplasm.
Two component system-ChiS sensor kinase senses chitobiose binding proteins. Empty chitobiose binding protein usually attached to ChiS sensor and maintains it in an ‘off’ position. CBP bound to chitobiose detaches it from ChiS.
ChiS is activates. His on helix passes Pi to aspartate. Pi passed to another His domain. Pi passed to asp on response regulator (ChiS~P).
What does ChiS~P do?
ChiS~P controls expression of ~ 200 genes.
Genes encoding chitinases, chitoporins, a special pilus, chitobiose transporters, chemotaxis components.
Pilus ensure bacteria stick to lobster and can’t be washed off.
What is carbon catabolite repression?
E. coli employs a ‘pecking order’ for carbon sources – it prefers some more than others!
Diauxic growth.
Mix bacteria, glucose, lactose together.
E.Coli use glucose first to grow.
When it runs out, lag phase in growth, will then use lactose to grow.
Called catabolite repression- physiology set up to repress genes for lactose utilisation until needed.
Shift is called diauxic growth.
