What's in YOUR papaya?



Papaya: The fruit with it's
own symposium



Some people out there are concerned over whether the food they are eating has been genetically modified (GM).  We don’t worry much about that in the Lab since there is nothing even remotely genetic about Twinkies or Mountain Dew.  Still, I mentioned before that a hackerspace was a great place to do some food surveillance.  During my research into traditional methods (PCR detection, etc), I ran across an article published a few years ago that described a really simple method to test for GM Papayas.  If you are a big papaya eater, this could be important, but the method could also be more general.

Papaya are susceptible to a nasty virus called Papaya Ringspot Virus (PRSV) which creates havoc in the fields.  Long ago, scientists found that by introducing a gene for the coat protein of this virus into the papaya genome, they could block the virus from attacking the plant.  However, another gene comes along for the ride – namely, a gene used for genetic selection in the lab.  When scientists introduce a gene into a plant cell, they often use a reporter to show them which clones successfully received the gene and which do not.  This marker is an enzyme called beta-glucuronidase (GUS).  What makes this such a good indicator of a GM papaya is that non-transgenic papaya do not have this enzyme, so if you can detect this in your fruit, it has to be transgenic.  One way to this, of course, is to use DNA primers specific for GUS and use PCR to try and amplify the DNA specific for the enzyme.  You could also try to sequence the DNA extracted from the papaya and look for the sequence for GUS (or any other inserted gene, such those for the PRSV coat proteins).  The complete genome of one type of GM papaya (the SunUp variety) was published in 2008 (Nature abstract) and is freely available at GenBank (link).  Others have also been subsequently published.



X-Gluc: A GUS substrate



Fortunately, for papaya lovers, there is an easier way.  5-bromo-4-chloro-3-indoyl glucuronide (X-Gluc) is a substrate for GUS and gets converted to glucuronic acid and a precipitate which happens to be blue.  Therefore, a very easy way to look for the presence of the enzyme is to screen directly with X-Gluc.  You don’t even have to purify DNA!  Simply take the seeds from the fruit of interest and smash them up really well.  Then incubate them in the presence of X-Gluc for about a day and if you see blue, you’ve got a GM papaya!  Here is a link to a recent article that describes the general idea (abstract).  In principle, if your hackerspace has access to X-Gluc, testing for the presence of GM papaya is extremely easy.  Scientists have used this same method to track cross-pollination between GM plants and non-transgenic neighbors.  One example was reported at the International Symposium on Papaya (abstract).  Yes, a papaya conference… and I thought I got a lot of grief for doing phage display.  Anyhow, I’ll have another post or two about this type of food monitoring and pretty soon you’ll be just like the USDA – only without the bureaucracy and bad jokes.