Fluorescence occurs when a substance absorbs a wavelength of light (often high-energy photons like violet or blue) and immediately re-emits that energy at a longer wavelength (green to red). Turn out the light and the “glow” goes away. Fluorescent materials are all around us, from the chlorophyll in leaves to some of the inks on our money, driver’s licenses, and magazines.
Try shining one of those blue keychain LEDs around your house and yard, while looking through a pair of yellow-lensed sunglasses (to block out the blue light and make the fluorescence more visible). Check out the mustard in the cupboard, and some white laundry.
Many animals have fluorescent structures; some of these have obvious functions, and others are there for unknown reasons, or they may fluoresce just as a side-effect of other chemical properties. One of the most famous molecules around (yes, there are famous molecules) is the green-fluorescent protein (GFP). What makes this protein so amazing is that you can put the gene (DNA sequence) that codes for it into just about anything, and when that organism or cell pumps out the protein, it becomes brightly fluorescent.
The gene for GFP was first pulled out of the jellyfish Aequorea victoria (image shown above). Later, using specimens bought at a Russian aquarium store, Mikhail Matz and his colleagues showed that corals also use a gene very similar to GFP to make green, yellow, and red fluorescence(1).
Now, about 100 fluorescent proteins (FPs) are known, and with this palette of colors at their disposal, biotech researchers are able to “paint” inside living organisms. With different colors, it possible to highlight (literally) two or three internal structures of interest, whether they are neurons or tumors, and follow their development over time, peeking inside without necessarily having to do a dissection.
One recent application of this fluorescent paintbox is both ridiculous and sublime: Jean Livet, Jeff Lichtman(2) and colleagues used fluorescent proteins to create multi-colored images of the neurons in a mouse brain. By using three or more FPs in combination, they claim to be able to produce up to 90 different tints (practically, maybe a dozen colors can be readily distinguished).
New fluorescent proteins are being discovered all the time, mostly from corals — not surprising since one piece of coral tissue may contain six or more different FP genes. Following the discovery in corals, FPs were found in copepods, little bug-like crustaceans that are like the insects of the sea. Now in October 2007, even more recently fluorescent proteins are making the news again with their discovery by Dimitri Deheyn and his colleagues in a strange organism called amphioxus.
Although they look like little eyeless fishes, amphioxus, or lancelets, are invertebrates. They don’t have a backbone, just a neural tube like the one in our spine. Thus they are one of the invertebrates that are closest to vertebrates — a far cry from a coral or jellyfish. It is not at all clear what function the fluorescence is serving in these animals, which spend most of their time burrowed in the sand.
So now FPs are known from cnidarians, near the base of the animal tree of life, and from amphioxus, lying on a remote branch. Because their DNA sequences are so similar, the chance that these fluorescent proteins evolved totally independently is vanishingly small. What this means is that there is a common evolutionary “ancestor” to the FP gene that is likely to be found across the animal kingdom. We should expect to discover more fluorescent proteins in ever more unlikely organisms.
How the gene was found in amphioxus is interesting in itself: As with corals, the fluorescence of amphioxus had been known for a long time, but nobody knew what caused it. Deheyn used the DNA sequence of a fluorescent protein from a coral, and searched for similar genes in amphioxus genome, which is publicly available online. Armchair science at its finest. (Of course, they went on to do many further experiments.) This is something that a student could do, given the right amount of insight and creative thinking! In fact, in a future report, I’ll describe how to search the growing number of available genomes to make your own amazing discoveries.
(1) One interesting aspect of this discovery is that scientists had known about the fluorescence of corals for about 50 years, and had the GFP sequence for 15 years, before the connection between the two was made. Lesson: nothing is too obvious to be true.
(2) I know I am not in a position to point out amusing connections between names and careers, but “Light-man” does seem appropriate for this particular study.