Reporters and GFP

In my previous post I explained that basic BioBrick parts are the building blocks used by synthetic biologists. Protein coding sequences are a key category of basic parts because proteins are the basis of biological systems. Reporters are an important family of protein coding parts used to "report out" to the researcher when a biological system has performed a particular function. Though there are many reporters available to synthetic biologists, including cell buoyancy, cell death, and antibiotic resistance, color is heavily utilized because it is easy to work with and color coding is an appealing way to label outcomes. Seeing red bacterial colonies on your Petri dish is a nice positive result—and it makes for a nice picture.

There is a large selection of color-based reporters available to synthetic biologists, but fluorescent proteins are a favorite to many—and for good reason. Fluorescent proteins are just that-proteins that fluoresce, or give off visible light when excited by ultraviolet light. And they do it on their own, without requiring small molecules like many other color reporters require. For example, fireflies use the enzyme luciferase to produce light, but it only works if fed a steady supply of the molecule luciferin. This is an annoyance because every culture must be supplemented with a proper dose of this expensive chemical, adding another variable to the experiment.

Fluorescent proteins, on the other hand, give off light by virtue of their chemistry alone. The most famous fluorescent protein is known as GFP, which simply stands for green fluorescent protein. GFP was first cloned from the jellyfish Aequorea victoria in 1994, and I can safely say that it has since made its way into all university molecular biology labs and a number of high schools as well. The utility of GFP is broad, but my research group uses it to signal when bacteria have a correct solution to a math problem (bacterial computation). If the BioBrick part E0040 (GFP) is incorporated into a biological system in the right way, then you can program your E. coli to glow green under specified conditions.

For example, it would be relatively straightforward to build a biological system in E. coli that responds to a dietary input by glowing green. Using the lac promoter (switch) to regulate the production of GFP would allow you to induce GFP production by adding the sugar lactose to your cultures. After a short time, as the GFP molecules accumulate inside the E. coli cells, the cultures would begin to glow green if illuminated with ultraviolet light. Likewise, the use of a promoter sensitive to lead would allow you to build a biosensor that glows green in the presence of dangerous levels of lead in drinking water. This versatility illustrates the strength of both the GFP reporter system and BioBrick parts.

GFP has a number of very crucial applications in fundamental research as well. Although the details are beyond the scope of this post, GFP can be fused with a native cellular protein to act as a sort of tracking device. The GFP is covalently linked to the native protein, and simply hangs off its side. Using a microscope, one can pinpoint where a particular type of protein "hangs out" in a cell by looking for green spots. And in multicellular organisms, GFP can be used to track in which tissues a particular protein is produced. This is a powerful tool for unraveling the functions of uncharacterized proteins, and the pictures that result are often quite amazing!

Stay tuned for my upcoming post about composite parts, plasmids, and more about the Registry of Standard Biological Parts.

Image Credit: Adapted from PDB file 1EMA; Figure 1 from Haddock, Rivers, and Robison; and a personal photograph.

Haddock, S. H. D., Rivers, T. J., and Robison, B. H. Can Coelenterates make Coelenterazine? Dietary Requirement for Luciferin in Cnidarian Bioluminescence. PNAS 98, 11148–11151 (2001).

References and Further Reading:

Goodsell, D. S. Molecule of the Month: Green Fluorescent Protein (GFP).

Zimmer, M. GFP: Green Fluorescent Protein.