The process of gene tagging — a commonly used technique for tracking genes in cell biology, molecular biology and biochemistry — is a technique created by Tsien, who won a Nobel Prize in 2008 for his discovery in green fluorescent protein (GFP). He created genetically engineered bacteria capable of producing differently-colored fluorescent proteins.

Scientists use these glowing proteins to effectively track the location and time when certain genes are expressed in cells, and to isolate specific proteins within organisms. To make a gene glow, gene coding for the GFP is fused with the gene for a fluorescent protein. This causes the GFP to glow, allowing scientists to track its location within a cell or an entire organism.

GFPs have many useful applications. Researchers can use these fluorescent proteins to highlight neural circuits within the brain, which could help researchers discover defecting circuitry that causes degenerative diseases like Parkinson’s and Alzhemier’s disease.

In 1995, Tsien determined how GFP became fluorescent, then his discovery to create mutant forms of GFP and differently-colored proteins. This single point mutation increased the effectiveness of GFP and broadened its spectrum of colors as well. Tsien spoke of the significance of having a range of proteins that each emits unique colors.

“Basically you can monitor many signals at once inside the cell, and you can monitor interactions of proteins by the phenomenon of fluorescence resonance energy transfer,” Tsien said in a Dec. 6, 2008 interview with Nobelprize.org.

Tsien initially had difficulty making the original wild-type GFP — the standard form of a species as it appears in nature — useful, because it was easily influenced by its environment. It was harvested from jellyfish.

“One remarkable property of the original jellyfish protein is that it actually isn’t just green. It excites, well it emits green, but it excites mostly in the UV, and only a small amount in the blue,” Tsien said. “And that’s a strange paradoxical phenomenon because we discovered that almost any mutation of one amino acid right next to the chromophore will shift it to being all of one or all of the other – either all UV or all blue.”
Scientists do not yet understand why the jellyfish proteins glow.

“One of the great mysteries that I’ve never figured out is why the jellyfish chose the only amino acid that would compromise, and be schizophrenic, and be partly UV and partly blue in its absorbance,” Tsien said. “And it could have so easily shifted it to all of one or all of the other. Instead it’s preserved this split character. We don’t know why.”

Scientists can manipulate amino acids to control what color they want a protein to emit.

“At least for the wild-type, there’s one amino acid that controls really primary color of the emission, and the next one to it controls, if you are going to be green, what is your absorbance spectrum,” Tsien said.