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Recombinant DNA procedures like those described here now have a number of practical applications. One of the most promising is the development of genetically modifi ed (GM) foods and agricul-tural products, substances whose genetic composition has been altered by rDNA or some similar process. Some examples of ge-netically modifi ed products are corn plants that have been altered to emit a poison when attacked by pests; tomato plants that spoil at a signifi cantly slower rate than natural plants; rape plants that have been engineered to be resistant to pesticides; and rice that has been enriched with a gene that codes for the production of vitamin A.

One of the fi rst GM agricultural products to be developed was a biological control agent named Frostban. The principle behind

the development of Frostban is that frost does not begin to form on plants in nature, even if the temperature is low enough, in the absence of certain frost-promoting bacteria. Frostban was a spray that contained other types of bacteria that had been genetically modifi ed to destroy the frost-promoting bacteria. When Frostban was sprayed on a crop, these genetically modifi ed bacteria attacked and destroyed frost-promoting bacteria, preventing plants from freezing even when the temperature was low enough for that process to occur normally.

The fi rst fi eld-testing of Frostban in 1986 was met by strong re-action from activists who were concerned about the unknown ef-fects the new product might have on the surrounding environment.

On the night before the test spray was to occur, these activists crawled through the fi eld where Frostban was to be tested and pulled up all the strawberry plants on which the test was to occur. In spite of this effort, the spray was eventually tested and found to be

Recombinant DNA procedures are commonly used to introduce pesticide resistance into a crop. (Maximilian Stock Ltd./Photo Researchers, Inc.)

effective. It was never produced on a large scale for commercial use, however.

The fi rst GM product to reach the marketplace was a tomato produced by Calgene, Inc., that was given the name of Flavr Savr.

The tomato was approved for use by the FDA on May 18, 1994. It was developed to provide consumers with fresher-tasting tomatoes during the winter months, when fresh tomatoes are generally not available. At the time, the only way for people in cold climates to get fresh tomatoes during the winter months was for those tomatoes to be picked while they were still green, allowing them to reach the marketplace before they began to rot.

To solve this problem Calgene scientists synthesized a copy of the gene in tomatoes that causes them to soften over time. That gene normally promotes the production of an enzyme, polygalac-turonase, that breaks down cell walls. The scientists then inserted the gene into tomato plants in reverse sequence (a process known as antisense insertion) so that it would produce an effect opposite that which it normally causes; that is, it inhibited the production of polygalacturonase, causing the tomatoes to soften more slowly than would normally be the case in nature. The advantage of the Flavr Savr tomato was that it could be left on the vine longer, until it had actually ripened, before being picked.

In 1991 Calgene asked the U.S. Food and Drug Administration (FDA) to review the company’s research on the Flavr Savr and de-termine whether it was safe for consumer sales. In May 1994, the FDA agreed that it was. It reported that Flavr Savr was “as safe as tomatoes bred by conventional means” and authorized Calgene to market its new product to the general public.

In spite of FDA’s approval, Flavr Savr tomatoes were never a com-mercial success. Some observers have blamed the product’s failure on consumer resistance (because the altered tomatoes cost too much or did not taste good enough). Others claimed that the company did not market the new product correctly or aggressively enough, and still oth-ers suggested that complaints about the use of GM products led to the product’s downfall. In any case, Calgene was bought out by the chemi-cal giant Monsanto in 1995 and shortly thereafter the parent company ceased research on and production of the Flavr Savr tomato.

A number of other GM foods have, however, met with greater com-mercial success than either Frostban or Flavr Savr. Some examples are golden rice and various Bt foods.

Golden rice is the name given to a genetically modifi ed food that supporters hope will solve vitamin A defi ciency (VAD), a widespread health problem in many developing nations. The most serious con-sequences of VAD are blindness and, in some cases, death. By some estimates, as many as 125 million children worldwide may suffer from VAD.

In the 1990s, the Swiss botanist Ingo Potrykus (1933– ) de-veloped a possible solution to the problem of VAD. Potrykus and his colleagues at the Institute of Plant Sciences at ETH Zürich (Eidgenössische Technische Hochschule Zürich) developed a modi-fi ed form of common rice seed that includes three additional genes:

psy (phytocene synthase), lcy (lycopene cyclase), and crt1 (phytoene desaturase). These genes have critical roles in the synthesis of beta-carotene, a precursor of vitamin A. Golden rice is so named because GM rice containing these three genes has a yellowish color because the lcy gene is obtained from the daffodil.

Like other GM food products, golden rice has its share of critics.

Some authorities suggest that children for whom golden rice is in-tended may not be able to digest, absorb, and convert the beta-caro-tene produced in the product. These children require an adequate, more complete diet that includes suffi cient levels of protein and fat if they are to benefi t from the engineered product. Some observ-ers suggest that much simpler solutions, such as providing a few teaspoons of red palm oil, readily available in many areas, would provide as much benefi t as that provided by golden rice. Proponents argue that no single food product can be expected to meet all the needs of hungry people in developing nations, but that golden rice has many attractive features that can help with at least one major health problem, VAD. (For an extended discussion of the pros and cons of golden rice, see “Golden Rice” at http://www.biotech-info.

net/golden.html.)

Bt corn is a genetically modifi ed food named for the organism from which the transmitted gene is taken, the common soil bacte-rium Bacillus thuringiensis. These bacteria produce proteins known

as delta endotoxins (or insecticidal crystal proteins [ICP]) that are toxic to a wide variety of insects. More than 150 insects belonging to the orders Lepidoptera (such as butterfl ies and moths), Diptera (fl ies), and Coleoptera (beetles) are known to be susceptible to the action of Bt delta endotoxins.

The mechanism of this action is now well understood. While in-tact, the proteins are nontoxic to the insect. When an insect ingests one of these proteins, however, it dissolves in the animal’s intesti-nal fl uid. Once the protein has dissolved, enzymes in the insect’s guts known as proteases attack it. These proteases break the protein