L33: V fischeri gene expression Flashcards
What are the two ways V fischeri genes are regulated
- catabolite repression because of starvation
- autoinducer control because of high cell density / QS
Describe the V fischeri gene operon
so the operon is the set of genes that are expressed on 1 mRNA
the left operon control LuxR
the right operon controls luxICDABEG
Explain the V. fischeri baseline operon expression conditions
cell are not starving and not high density
left operon not expressed
right operon expressed at low levels cause there is always a little bit of luxI (consituitive expression) meaning the cells always make a little bit of AI
this low level is not enough for light production but enough for quorom sensing
Explain V fischeri catabolite repression, explain in other bacteria too
The control of gene expression is done by [cAMP-CRP] in response to the availability of catabolic nutrients
=> it induces or represses gene expression depending on where it bidns in the promoter of the gene under regulation
cAMP-CRP regulates many genes in ecoli and other bacteria too, lac operon in ecoli is the model system
in most chemoorganoheterotrophic
What are catabolites, and what is repression
– catabolites are nutrients metabolized to generate energy
– repression is a reduction in gene expression
Explain control of lux gene expression by cAMP-CRP
the lux genes are only expressed if the fish are starving and starvation is signalled by high cAMP-CRP []
high cell density will always leads to starvation
high nutrient supply can sustain the a high cell density of V fishy but cannot prevent the cells from sensing the starvation condition
=> so they may be ‘‘starving’’ but still receive enough nutrients to make lightt
break down the cAMP - CRP and its components
cAMP
* reg compound derived from aTP, increase in starving cells and is effective at low concentrations
CRP
*catabolite repression protein, gene reg protein activated by binding to cAMP
together
*can bind to gene promoters and turn expression of genes on or off
*the [cAMP-CRP] is proportional to [cAMP]
so all together, [1-2] will increase in starving cells and mediate expression so the cells can respond to starvation
Explain concentrations when genes are not starving
- cells are not starving
– low [cAMP-CRP]
– lux genes are not expressed: no light is emitted
Explain concentrations if nutrients are scarce
– cells are starving
– high [cAMP-CRP]
– cAMP-CRP binds to DNA at the luxR gene promoter (OL this will results in luxR gene expression and synthesis of the LuxR protein and LuxR will then be available to bind to autoinducer (AI)
– high cell density is determined by high [AI] and quorum sensing
Explain Autoinducer control in terms of QS
– [AI] only reaches high enough level to activate sufficient LuxR to for light production when cell density is -10^9 - 10^10 cells per ml
– a threshold [AI-LuxR] is reached at this high cell density
then AI-LuxR binds to the luxI promoter DNA (OR )
this causes the luxICDABEG operon to be expressed
then light emission is a by-product of luciferase reaction
Explain Autoinducer control in terms of AI/ autoamplification
autoinduction: luxI gene expression is controlled by its gene product (AI)
– amplification of lux expression occurs as [AI-LuxR] increases => more & more light until sufficient for counterillumination
– repression of lux expression occurs at very high [AI-LuxR] this is controlled, energy efficient: steady, sufficient ligh
How does v fishy control over expression
repression of lux expression occurs at very high [AI-LuxR]
this results in less LuxR, less AI-LuxR, less induction of OR with then results in decreasing concentration of AI
– prevents over-expression of system => allows efficient maintenance of a delicate balance also recall that squid can also adjust amount of bioluminescence
in response to intensity of downwelling light (via lensing
and shielding via ink sac (…etc?))
– while the cells are starving, they continue to receive
enough nutrients from the animal to fuel the luciferase reaction
– light organ is able to produce light until most of the Vibrio are
expelled in the morning
