I was overflowing with excitement as I walked in
to biology class. I couldn't wait to raise my glowing plate of bacteria in
scientific triumph!
We had been so careful, so precise...
But it was all for naught.
Alas, my LB/Amp/Ara plate, like so many others before me,
was utterly blank. There was not a SINGLE speck of glowing e. coli to
give me any semblance of satisfaction. And thus, rather than expand upon
the elusive causes behind my failures, I will use this blog post to explore
some of the finer points of this otherwise fascinating lab.
I wanted to learn more about the function and mechanism of
the green fluorescent protein (GFP) responsible for
the fluorescence of our e. coli specimens transformed with the pglo
plasmid. GFP was discovered by Osamu Shimomura, Martin Chalfie, and Roger Tsien
in the 1960's and 1970's. The first reported crystal structure of a GFP in
1996. Chalfie, Shimomura and Tsien shared the
2008 Nobel Prize in Chemistry for their discovery and development of
the green fluorescent protein. In fact, I was lucky enough to meet Martin
Chalfie at the Intel International Science and Engineering Fair in
Los Angeles.
GFP has a unique soda can shape. Inside the can is the
chromophore (the part of a molecule responsible for
its color). Thus, GFP is occasionally referred to as the
“light in the can.”The chromophore produces the
fluorescence. It consists of three peptides
consisting of the residues serine, tyrosine, and glycine at positions 65-67 in
the sequence. Although this simple amino acid sequence is commonly found, it
does not generally result in fluorescence because the structure of the entire
protein is necessary for the reaction. Upon formation of the polypeptide chain
of GFP, the folding and other reactions occurs automatically and no activator
is necessary for fluorescence. However, I still wanted to know exactly how
this protein glowed. As usual, I found it was a lot more complicated that I
would have thought.
"Fluorescence (indicated by a green glow surrounding
the affected structural elements) occurs when oxidation of the tyrosine alpha-beta carbon
bond by molecular oxygen extends electron conjugation of the imidazoline ring
system to include the tyrosine phenyl ring and its para-oxygen substituent. The
result is a highly conjugated pi-electron resonance system that largely
accounts for the spectroscopic properties of the protein."
(source http://www.olympusconfocal.com/java/fpfluorophores/gfpfluorophore/index.html). This
is illustrated in the figure below.
A. The molecular structure of Aequorea green
fluorescent protein as viewed from the top
B. A proposed mechanism for the series of post-translational
modifications that converts the serine 65, tyrosine 66, glycine 67 tripeptide
sequence into the fluorescent chromophore (Heim et al. 1994).
Apparently flourescent proteins are found in over 125
species. I couldn't help but to wonder what was the point? UV light from the
sun is not strong enough to cause these proteins to light up and it's not as if
animals are carrying around UV lights.
GFP typically functions in many bioluminescent organisms
(which is quite different that florescence - Bioluminescence is a
naturally occurring form of chemiluminescence where energy is
released by a chemical reaction in the form of light emission.). GFP acts as
a bioluminescence resonance energy transfer (BRET) acceptor. These
convert the blue emission of the bioluminescent protein into a longer
wavelength green emission. OK, great. So what it the point of that?
We've finally come to the end of absolute answers.
Biologists aren't really sure about the point of BRET molecules. One theory is
the "Burglar-alarm" hypothesis in which the jelly fish will
light up upon being attacked to attract secondary predators. The glowing
jellyfish typically only light up after stimulation or stress.
Florescent proteins have other function in Anthozoans
(stony corals). One study suggests they provide photoprotection to
symbiotic photosynthetic algae living inside the corals. Another theory is that
the range and patterns of coloring due to fluorescent proteins may help
reef fish identify different species of corals.
My research indicated that there are numerous speculative
hypotheses regarding the biological roles of fluorescent proteins. As one site
put it "It is remarkable that, despite the critical role of coral
reefs in supporting a wide diversity of ocean life and providing food for
humans, so little research has been devoted to understanding the role of color
and fluorescence in reef biology and ecology."
A science fair judge (at least a good one) would not be so
happy with this lab - namely its experimental design. At the end of
the experiment (a successful one), the only thing we know for sure is
that the transformed bacteria become ampicillin resistant while the untransformed
bacteria are not ampicillin resistant. However, the interesting thing
is the glowing part of the experiment is not well controlled because only 1 of
the 4 plates has arabanose. For instance, what if the original bacteria
actually glowed. We wouldn't know because we never but them on an ara plate
unless they had been transformed. Were the creators of the lab trying to give
us something to write about in our lab reports or were they simply
setting a bad example?
Overall, the 2 -pglo plates functioned as the supposed
control and the +pglo were our variable group. The variable was the
addition of pglo plasmids. It's fascinating to me, however, that we never
tested out our original bacteria in the ARA or AMP plates. Perhaps they started
out as amp resistant and glowing. Maybe our procedure of heating and cooling
destroyed those characteristics. Who knows? The point however is that it wasn't
that well controlled (though there is a cost factor to be considered, which I
understand Mr. Wong)
Rather than mindlessly plod through a procedure, I would
rather understand why we did what did (which I'm sure
most people didn't)
transformation solution (CaC12) - the positive charges of
the Ca+2 ion neutralizes the negative DNA phosphates and negative charge of
membrane phospholipids.
Ice treatment - slows the fluidity of the cell membranes
Heat shock - increases permeability of the cell membrane
So who cares about a stupid glowing protein? It turns out a
lot of people. GFP has a vast range of important applications. Rather than go through
all those, I'm just going to look at one. The "Brainbow" mouse was an
experiment in which each cell of a living mouse's brain was colored one of
about 90 different colors. The colored neurons will help simply the complex
tangle of neurons that make up the brain and nervous system: a sort of circuit
diagram if you will. Use of the protein is important as these cells are not simply dyed different colors. They are randomly expressing different colors of florescent protein. See pretty pictures below: