When scientists use transgenic technology to put a new gene into a plant, they put in additional pieces of DNA to direct the activity of that gene. Each gene needs a "promoter" to turn it on under specified conditions. The most widely used promoter is the cauliflower mosaic virus 35S promoter, often abbreviated as the CaMV promoter or the 35S promoter. This promoter was obtained from the virus that causes cauliflower mosaic disease in several vegetables, such as cauliflower, broccoli, cabbage, and canola.

Other promoters have been used in developing crops, but the CaMV promoter is often chosen because it causes abundant production of the transgenic protein in a wide variety of situations. These characteristics, which are seen as advantages for many purposes in transgenic technology, have raised concerns that the CaMV promoter might be harmful if it were to invade our cells and turn on our genes. A multi-step chain of events would have to occur for the CaMV promoter to escape the normal digestive breakdown process, penetrate a cell of the body, and insert itself into a human chromosome. There is some evidence that fragments of DNA sneak into the blood stream and travel to some internal organs, and that these fragments sometimes become closely associated with host DNA. For a discussion of this, see our segment on eating DNA. There have been no tests to determine whether the CaMV promoter has invaded human tissues or whether fragments of it would include enough of the genetic code to function as a promoter.

Canola leaf infected with cauliflower mosaic virus. Source: Institut National de la Recherche Agronomique, Versailles-Grignon

Research on plasmid DNA in rice by Kohli et al. (1999) has been cited as evidence that the CaMV promoter can insert itself into strands of DNA. Opponents of transgenic crops provide a full discussion of the viewpoint that this poses a threat to people at http://www.btinternet.com/~nlpwessex/Documents/camv.htm.

Opponents of transgenic crops discuss the details from the Kohli paper at http://www.netlink.de/gen/Zeitung/1999/990715.htm.

Kohli did not study the behavior of the CaMV promoter in human cells or animal cells. For an opinion that insertion of the CaMV promoter by itself into other genomes is unlikely, see http://www.foodsafetynetwork.ca/gmo/camv35s/camv35s.htm.

Leaf infected with cauliflower mosaic virus. Source: Institut National de la Recherche Agronomique, Versailles-Grignon

Human chromosomes contain the remnants of DNA sequences from what appear to have been many different viruses. The frequency of such sequences in human chromosomes is interesting and leads to speculation about what these sequences would do if they were activated. But research by Turner et al. (2001) indicates that most of these sequences are nonfunctional because of multiple internal changes that have occurred over thousands of years. They probably would not be able to do anything even if they were activated by the insertion of the CaMV promoter (Royal Society, 2002). There is some evidence that the CaMV promoter poses little threat to human health. People have been eating it in small quantities for hundreds of years when we eat vegetables that are infected with the disease. Although vegetables heavily infected with CaMV are unappetizing, there have been no documented negative effects on health from eating the virus or its promoter.

For an opinion that eating the cauliflower mosaic virus promoter poses no threat to human health, see the Royal Society's 2002 report "Genetically modified plants for food use and human health--an update" (http://www.royalsoc.ac.uk/files/statfiles/document-165.pdf).