What are the 2 regulators of light production by Vibrio fischeri?
- Catabolite Repression => in response to starvation
2. Autoinducer Control => in response to high cell density (quorum sensing)
What is an operon?
A set of genes hat are expressed on a single mRNA
What do the left and righ operon encode?
Left operon => encodes LuxR
Right operon=> encodes luxICDABEF
What is the baseline expression of V.fischeri lux operon?
- Left operon normally not expressed
- Right operon normally expressed at a very low level (constitutive expression)
- Cells always make a little bit of AI
- Or expression not high enough for light production
Explain catabolite repression
- The control of gene expression by [cAMP-CRP] that occurs in response to the availability of certain catabolic nutrients
- Model system for the study of catabolite repression is the LAC OPERON of E.Coli
- Found in many if not all chemoorganoheterotrophic bacteria, but note that different organisms utilize different catabolites
- Catabolites are NUTRIENTS metabolized to generate energy
- Repression is a reduction in gene expression
How is the expression of lux gene controlled by cAMP-CRP?
- Lux genes can only expressed if V.fischeri cells are starving
- Starvation signal conveyed via [cAMP-CRP] - Starvation is an inevitable consequence of VERY HIGH CELL DENSITY in the squid light organ
- The constant supply of nutrients from the squid host is sufficient to allow growth of a V. Fischeri population to high density and maintain a high population density for a while, but insufficient to prevent starvation.
- Even though V.fischeri cells are “starving”, they receive enough nutrients to survice and to fuel light production
cAMP is a regulatory compound derived from ATP
- Increases in starving cells
CRP (catabolite repression protein )
- Gene regulatory protein activated by binding to cAMP
cAMP-CRP
- Can bind to gene promoters and thus turn expression of genes on or off when concentration high enough
If nutrients are abundant….
- Cells are NOT starving
- LOW [cAMP-CRP]
- Lux genes are not expressed : NO LIGHT is emitted
If nutrients are scarce…
- Cells are STARVING
- HIGH [cAMP-CRP]
- [cAMP-CRP] binds to DNA at the luxR gene promoter (OL)
- Results in luxR gene expression and syntehsis of the LUXR protein
- LUX R will then be available to bind to autoinducer (AI)
- Results in luxR gene expression and syntehsis of the LUXR protein
- High cell density is determind by high [AI]
- Quorum sensing
Explain the autoinducer control of light emission
Quorum sensing
- [AI] only reaches high enough level to activate sufficient LuxR for light production when cell density is 10^9-10^10 cells /mL
- A treshold [AI-LuxR] is reached at this high cell density
- AI-LuxR binds to the luxI promoter DNA (Or)
- Causes the luxICDABEF operon to be expressed
- Light emission is a by-product of Luciferase reaction
Why is it called autoinducer control?
- LuxI gene expressiosn is controlled by its gene product (AI)
How does amplification of Lux expression occur?
Amplification occurs as [AI-LuxR] increases
- More and more light until sufficient for counterillumination
When does repression of lux expression occur?
Repression of Lux expression occurs at VERY HIGH [AI-LuxR]
- Results in less LuxR, less AI-LuxR, less induction of O=> results in decreasing concentration of AI
- Prevents over-expression of system
- Allows efficient maintenance of a delicate balance
- Light organ will still be able to produce light until Vibrio are exprelled in the morning
- Steady, sufficient light, energy efficiency
