Safe Food: The Politics of Food Safety (53 page)

• Sequences of DNA bases (DNA segments) arranged in a specified order constitute genes.

• Some gene DNA sequences specify the structure of proteins.

• Other DNA sequences specify the structure of molecules that signal where genes begin and end.

• Gene DNA sequences specify the order in which amino acids link to make specific proteins; a sequence of three DNA bases specifies 1 of the 20 amino acids (this is the
genetic code
).

• Proteins are composed of various combinations of the 20 different amino acids linked in a specific order defined by the gene DNA sequence.

• Proteins do the work of cells, muscles, and other organs as structural components, signals, or enzymes.

• Enzymes catalyze biochemical reactions in the body.

• The structure of DNA is helical; its two strands are twisted around each other in a double helix.

• Proteins differ from one another in structure; they fold into specific three-dimensional shapes that depend on the sequence of their amino acids (and other components that may be introduced during or after protein synthesis).

• The structure of a protein determines its function.

These biological features operate in the same way in most organisms. Differences among species depend on the specific order of base sequences in their DNA and, therefore, in the sequence of amino acids in their proteins. When scientists extract genes from bacteria, they are taking segments of DNA that contain the same DNA bases that are already in plants—just arranged in a different sequence. The commonality of DNA bases among organisms is the main reason why many scientists are perplexed by public anxieties about genetic engineering; DNA is DNA—its base subunits are the same—no matter where it comes from or where it goes.

As noted in
chapter 5
, the genetic engineering of beta-carotene into rice represents an extraordinary technical achievement. The “foreign” genes must be identified and reproduced, inserted into the plant’s DNA, and made to function in
the plant and reproduce in its seeds. How all of this is accomplished is quite remarkable, as the methods take advantage of the unique and rather bizarre properties of a species of common soil bacteria,
Agrobacterium tumifaciens
.
Table 17
outlines the use of this system to put genes for beta-carotene into rice; it describes the
less
complicated of the two approaches used for this purpose.
^3
,
4

Agrobacteria
can infect a variety of plants that have been scratched, torn, or “wounded” in some way. At the wound site, the bacteria induce the plant to form swellings—crown galls—a form of plant cancer. The bacteria do not actually penetrate into the plant’s tissues. Instead, they attach to the wound site and transfer a special piece of their DNA into the plant. This piece, called transfer-DNA (T-DNA), contains genes and DNA base sequences that enable it to enter the plant cells, find the plant’s DNA, integrate into it, and specify the production of proteins that cause plant cells to make crown galls. Why might
Agrobacterium
do this? The most likely explanation is that the T-DNA also contains genes that cause crown galls to produce unusual amino acid derivatives called opines. Opines are not normally made by plants and do nothing for them. Instead, they serve as a preferential food for
Agrobacteria
, giving them a competitive edge in the ecological world of soil bacteria.

What makes
Agrobacterium tumifaciens
uniquely qualified to transfer genes from other organisms to plants is that the T-DNA is not really part of its own DNA. Instead, the T-DNA is carried on a small, entirely separate, circular piece of DNA called a
plasmid
. Most bacteria contain plasmids (but without T-DNA). Plasmids are self-replicating, which means that they contain genes that specify their own reproductive functions; they multiply independently of the bacterial chromosome—the structure that contains the bacteria’s DNA.

Typically, plasmids carry genes for traits that are useful—but not essential—for bacterial growth or reproduction.
Agrobacterium tumifaciens
plasmids, for example, carry T-DNA and its genes for crown gall. Other bacteria contain plasmids with genes for a variety of functions highly germane to issues discussed in this book: the ability to fix atmospheric nitrogen, synthesize the
Bacillus thuringiensis
(
Bt
) toxin, produce pathogenic toxins (
E. coli
O157:H7 and
Bacillus anthracis
), resist certain antibiotics, and—most important—infect other bacteria. Plasmid genes for these last two characteristics, for example, are often responsible for the widespread dissemination of resistance to antibiotics within a bacterial species, and from one kind of bacteria to another.

Agrobacterium
plasmids are unique in containing T-DNA. On these plasmids, the T-DNA is flanked by DNA base sequences that mark its borders. As the T-DNA enters the plant, any DNA that lies between its border regions will be transferred into the plant’s cells, regardless of where that DNA came from.
Agrobacterium
plasmids, therefore, solve a major technical problem: how to get desirable genes from bacteria or other foreign sources inserted into the cells of food plants.

Plant biotechnologists select the genes they want from any organism, get rid of unwanted T-DNA genes responsible for crown gall and opines, insert desired genes and regulatory DNA sequences between the T-DNA border regions, and use the
Agrobacterium
system to inject the newly constructed T-DNA into plant cells. This system does not work efficiently, and only a rare plant accepts the T-DNA. To identify the successful transfers, scientists add marker genes to the T-DNA, usually for resistance to antibiotics. The constructed plasmid—with the original genes for infectivity (but with crown gall functions removed), and the desired genes, regulatory elements, and markers inserted into the T-DNA—is called a
transmission vector
. When the system works, the bacteria containing the vector attach to the plant and actively transfer the T-DNA to the plant’s cells. Once in the plant, the T-DNA genes and sequences integrate into the plant’s DNA; the integrated genes specify the production of the desired proteins; the proteins move to the appropriate places in the plant’s cells; and the plant displays the new characteristic.
⁵

TABLE 17.
Highlights of one of the methods used to genetically engineer beta-carotene into Golden Rice
*

SOURCE:
Ye X, et al.
Science
2000;287:303–305.

*
Refer to
figure 13
,
page 156
.

But that is not all. Constructing T-DNA sequences with foreign genes that actually function in plants requires the action of numerous enzymes that break DNA molecules at specific sites (“restriction” enzymes), enzymes that reattach split pieces (ligases), and a great many steps carried out in a specific order. For the system to work in rice, for example, the scientists also must successfully grow rice cells in tissue culture (an artificial medium containing nutrients and growth factors), infect the rice cells, grow them back into rice plants, and have the rice breed true under greenhouse conditions. Each one of these steps presents its own set of technical difficulties. Thus, genetic engineering requires a “feel” for how to make all of the steps work, which transforms the technology into an art as well as a science. The artistic aspects add to the difficulty of explaining the science to nonspecialists.

At issue is what is to be done to bridge the gap in knowledge and outlook between scientists and nonscientists. In a preliminary draft of this appendix (now much revised), I argued that scientists must work harder to explain their methods, approaches, and findings to the public, and that the public must take responsibility
for demanding such explanations. One of the scientists who commented on that draft said that if I am asking people to demand explanations, I must also insist that they
listen
to the explanations, and with an open mind. He also mentioned that people like me who attempt to provide understandable explanations of science have a responsibility to ensure that the explanations are reasonably complete. I have tried to do this but have also tried to explain the ways in which science is political and inextricably linked to its social context and consequences.

This section contains reference citations along with occasional notes. Citations follow the spare, unpunctuated “Vancouver” style used by most biological science journals, as described in
JAMA
1993;269:2282–2286 (translation:
Journal of the American Medical Association
, 1993, volume 269, pages 2282 to 2286). Sometimes, issue numbers follow the volume in parentheses. Thus,
Food Technology
1991;45(5):248–253 refers to an article published in the fifth (in this case, May) issue. As is customary in this style, text citations sometimes appear out of numerical order; these are space-saving cross-references to material cited earlier in the
same
chapter. Also to save space, references to multiple quotations or facts in a paragraph are listed in order under one note at its end; references to U.S. government reports omit their place and publisher (Washington, DC: U.S. Government Printing Office); and citations to articles in professional journals signed by multiple authors list only the first three followed by et al. Except as otherwise noted, documents obtained from Internet sources were available at the cited addresses in February 2010.

For clarity, most references give the full name of organizations, government agencies, and the titles of journals and publications, but certain frequently used terms are abbreviated as follows: