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Team:Exeter/Project
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Introduction
Our project aims to produce a Red, Green and Blue ([http://en.wikipedia.org/wiki/Additive_color RGB]) light control brick for E. coli. To demonstrate the system we will control the production of Cyan, Magenta and Yellow ([http://en.wikipedia.org/wiki/Subtractive_color CMY]) pigments to produce the world's first colour bio-photograph or [http://parts.igem.org/Coliroid coliroid].
We will be building upon the work done by previous iGEM teams, most notably Texas/Austin 2004, Edinburgh 2010 and Uppsala 2011.
The Camera
Bio-Photography is the application of genetically engineered bacteria as the light sensor of a camera, replacing digital sensors (CCD/CMOS) or photographic film. The surface area of bacteria is on the order of microns, much smaller than a digital sensor. This gives bio-cameras the potential to produce images with far greater resolution.
We aim to produce the world's first colour bio-photograph by producing Cyan, Magenta and Yellow pigments in response to Red, Green and Blue light. We plan to combine this system with a lens to focus the image onto plated bacteria to form the world's first colour bio-camera.
Colour system
To produce the full colour spectrum requires all three [http://en.wikipedia.org/wiki/Primary_color primary colours]. This is because any colour can be produced by mixing the primary colours but primary colours cannot be produced by mixing any colours.
In our system we are using two sets of primary colours. Red, Green and Blue for light and Cyan, Magenta and Yellow for pigments. This is because light and pigment mix differently; light is additive and pigment is subtractive:
Light and pigment produce colour in different ways. While coloured light has a peak wavelength corrosponding to its colour, coloured pigment absorbs all wavelengths that do not correspond to its colour and so reflect the corrosponding coloured light:
In this system Red is the opposite of cyan, green opposite magenta and blue opposite yellow. Hence our choice of sensors and pigments.
It must be noted that while [http://en.wikipedia.org/wiki/RGB_color_model RGB] light give a full spectrum of colour and tone. CMY is a simplified version of the CYMK colour system used in printing. This was done because to engineer the pathways for[http://en.wikipedia.org/wiki/CMYK_color_model CYMK] colour system outputs is too complex. however as CMY is subtractive there will be tone as Cyan + Magenta and Yellow will approximate black.
The Biology
The focus of our wet work is the creation of three independent 'light to output' pathways in E. coli. Each pathway will control the transcription of selected genes using Red, Green or Blue light. Incident light will prevent the transcription of the selected gene.
In our bio-camera the controlled genes will be those that code for the production of cyan, magenta and yellow pigments:
- Red light prevents the production of cyan pigment
- Green light prevents the production of magenta pigment
- Blue light prevents the production of Yellow pigment
Each sensor prevents the production of its opposing pigment, producing the pigment which is the same colour of light that the light sensor detects. The mix of pigments will reproduce the colour of light incident on the bacteria.
Red light pathway
This pathway uses the well characterised Cph8 red light sensor (composing of Cph1 and Envz) which acts to phosphorylate OmpR to OmpRp in the absence of red light. OmpRp then binds to the OmpF promoter upstream of the HO-pcyA gene which codes for cyan pigment, and stop codon. This is then transcribed and cyan pigment protein produced.
In the presence of red light, OmpR is not phosphorylated which means it cannot bind to the OmpF promoter, so the cyan pigment is not produced. When this occurs, cyan pigment will not be made, and a red pigment will be produced.
Green light pathway
The green light module works differently to the red and blye light modules but it still uses a light sensor (CcaS), intermediate protein (CcaR) and a specialised promoter. However, when the system is exposed to green light, CcaR is phosphorylated and CcaR-P is generated, allowing the synthesis of the magenta pigment to be up regulated instead of prevented. To make an overall green colour, we only want the yellow and cyan pigments to be transcribed, so production of the magenta pigment is problematic.
To overcome this issue, we will introduce an inverter system. CcaR-P will be produced when the bacteria are exposed to green light, but instead of binding to a promoter placed before the gene coding for magenta pigment, it will bind to a cI repressor system. The subsequent transcription of the cI repressor protein will prevent the synthesis of the magenta pigment, as it will bind to a cI promoter. Binding of cI to this promoter ceases transcription of any following genes, therefore magenta will not be produced.
Blue light pathway
The Blue light pathway works in much the same way as the red light sensor but FixJ is used as an intermediate instead of OmpR. The YtvA/FixL blue light sensor phosphorylates FixJ to Fixjp in the absence of blue light. FixJp then binds to the promoter for the Yellow pigment gene and stop codon. So in the absence of blue light the Yellow pigment is produced.
In the presence of blue light, FixJ is not phosphorylated which means it cannot bind to the promoter for yellow pigment so no yellow pigment will be produced. When this is the case, yellow pigment will not be made, and a blue pigment will be produced throught the mix of cyan and magenta.
The future
Our project aims to provide a foundation for the control of organisms using multiple wavelengths of light for future development. Using lasers to control bacteria will provide high spatial control while a well characterised system will provide good temporal control. This technology has many implications for the future.
