Thursday, March 28, 2024
University of Hawaii at Manoa
University of Hawaii System
College of Tropical Agriculture and Human Resources
 







GUS assay plate: Blue color indicates positive for GE (genetically engineered) plant

What is the GUS assay?

When we receive your leaf sample, we perform a laboratory assay called 'GUS' to determine if the plant is genetically engineered (GE).  
 
GUS assay. GUS is short for “beta-glucuronidase”, an enzyme common in bacteria, but not found in papaya plants. The presence of GUS in a papaya plant is evidence that it was put there by the same genetic engineering event that introduced resistance to the serious papaya disease called papaya ringspot virus (PRSV). PRSV resistance was the objective in genetically engineering papaya.  The GUS gene was included only to provide a quick way to distinguish the genetically engineered papayas from normal ones, since the presence of GUS is easy to detect in plant tissues by a simple color-change assay. 

The test produces a blue color in GE papaya leaves, but causes no color change in normal papaya leaves. The biological function of GUS in living cells is to remove or attach a sugar molecule in reactions with any of several larger molecules.  When the larger molecule is a dye supplied as part of the papaya-leaf assay procedure, removal of the sugar molecule by GUS enzyme changes the color of the dye from colorless to bright blue.  The development of blue color in the assay is a “positive” result, indicating the presence of genetically engineered GUS.  A colorless assay is a “negative” result, indicating a normal papaya lacking GUS.
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