Transfected cells
This is a photomicrograph of some human
embryonic kidney cells (HEK 293 cells) that I am growing. Notice the three
cells near the center which display a greenish glow. The cells are glowing
because they have been transfected with the cDNA from
a jellyfish, Aequorea victoria,
that codes for a protein called Green Fluorescent Protein (GFP). GFP is a
naturally fluorescent protein; when GFP is excited with blue light at 470 nm,
it emits green light at 509 nm.
The cDNA for GFP
was obtained from a commercial biotech supplier (Clontech).
I subcloned the jellyfish cDNA
into a mammalian expression plasmid, pCEP4 (Invitrogen).
This new DNA vector construct allows for the constituitive
expression of the GFP in human cells, driven by the cytomegalovirus promoter
sequence already present in pCEP4. The HEK 293 cells were exposed to a fine
precipitate of the GFP/pCEP4 DNA construct for 24 hours, then the DNA was
washed out. During this time, some cells take up the DNA, and begin to
synthesize the new GFP protein. After another 24 hours, I placed the cells
under a microscope equipped with epifluorescence
optics for fluorescein isothiocyanate,
illuminated them with blue light, and took this picture. Cells that took up the
GFP/pCEP4 construct fluoresce green, while untransfected cells do not.
The green fluorescence serves as a marker for transfection efficiency. When cells are co-transfected with GFP and some other DNA, for example receptor DNA whose protein product does not glow, I can monitor the success and extent of transfection by using the green fluorescence as a reporter.
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