Team:Carnegie Mellon

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Carnegie Mellon iGEM 2012


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Quantitative In Vivo Fluorescent Biosensors

A novel reporter system for synthetic biology



Image:Cmu2.jpeg

 

Contents

Introduction: Motivation and Background

What is fluorescence, exactly?

Fluorescence is a property of some molecules, particularly aromatic organic dyes that absorb photons at a certain wavelength and emit them at a longer, lower energy wavelength.

Fluorescence is described using quantum mechanics principles and organic chemistry. Five and six-member rings tend to fluoresce brightly because of electron delocalization and the properties that are associated with electron delocalization through the p-orbitals. Fluorescent molecules come in a variety of flavors and uses based on their properties, and shape. Fluorescent molecules are known as fluorophores and can take the form of organic dyes or proteins. So far, many different types of fluorophores have been discovered, developed and studied in great detail. Typically, fluorescent proteins have a fluorophore that consists of a few side chains that react and form a complex similar to that of an organic dye. For example, GFP (the most common fluorescent protein) has an HBI fluorophore. Our Spinach construct binds to a dye that derives from this fluorophore. Fluorescence of a molecule can depend on conformation, in the case of our fluorogen, malachite green, which is a conditional fluorophore, the molecule must be in a certain conformation to fluoresce, otherwise, it will absorb photons, but it will emit them very inefficiently (extremely low quantum yield). Fluorescence is a widely studied phenomena and a lot of research is involved with improving current fluorescence technologies and its applications.


What is Spinach?

Spinach

Spinach is an RNA sequence that can be expressed in cells (in this case, E. coli ) and fluoresces green when DFHBI (an organic dye) is bound to it that can be used to quantify RNA concentration in a cell. Spinach binds to an organic dye called DFHBI which doesn't fluoresce by itself but fluoresces very brightly when it is bound to Spinach. DFHBI is chemically derived from the chromophore in GFP but is altered to increase brightness when bound to RNA. Other fluorescent RNAs have been described but many are non-specific and have many unwanted functions like cytotoxicity. Other fluorescent RNAs also are difficult to quantify because the cells' RNAses (RNA destroying enzymes) can cut out the fluorescent sequence at unpredictable times, making quantification impractical. Spinach utilizes a scaffold that derives from a tRNA sequence, which disguises the RNA so that RNAses leave it alone. Manipulations to the sequence that Spinach is attached to allows for a variety of analyses functions. RNA can be arranged to bind to just about any small molecule in the same way that Spinach was developed (using a SELEX method) to track cellular metabolites. This allows for quantification of another important system in cells. However, in our system, Spinach is incorporated in the mRNA (between the promoter and the RBS) so mRNA is quantified. Click for more information on Spinach. Spinach is the first published RNA sequence of its kind and more sequence/dye combinations are in development; as a result, in years to come, multiple genes (both RNA and protein) can be analyzed in great detail simultaneously.

What is a FAP?

A fluorogen activating protein is a small (26-35kD) protein that derives from the variable region in an antibody. FAPs are not fluorescent unless a fluorogen (also not normally fluorescent) is added, in which case the FAP changes the conformation of the fluorogen and the complex fluoresces brightly. FAPs are currently used to tag certain proteins like actin or tubulin in mammalian cells. FAPs are not primarily expressed in E. coli although we have expressed certain FAPs in E. coli. The two main dyes that the current series of FAPs bind to are malachite green and thiazole orange; our construct uses a variant that binds to malachite green. These dyes are normally cell impermeable but can be designed to penetrate cell membranes. As a result, they were originally used to tag surface proteins. FAPs are excellent reporters because they are small proteins that are soluble and have virtually no maturation time and are highly photostable unlike traditional variants of GFP. FAP technology is widely unexplored but shows promise for new fluorescent technology. FAPs have been used to track individual molecules to 5nm definition as opposed to the typical 200nm. Engineered dyes, called dyedrons, have been developed that increase fluorescence intensity and can allow researchers to improve on live cell imaging techniques. FAPs are genetically different and respond to different excitation wavelengths so researchers can image multiple proteins at the same time in order to understand complex biological processes.

Why is this project important?

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