GMO Myths and Truths

spliced into or around the gene for various purposes. Most prominent among the genetic
control elements that are spliced to the gene of interest are “promoter” and “termination”
sequences.
The promoter marks the beginning of the gene. It attracts and binds multi-protein
complexes, called the gene expression machinery. This machinery reads the DNA sequence
of the gene and synthesizes a complementary messenger RNA (mRNA) copy of the gene
sequence. The termination element, as the name implies, marks the end of the gene and
causes the synthesis process to stop.
Promoter and termination elements must be sourced from organisms that will allow them
to work in the GM plant. These can be from either plants or, more frequently, plant viruses
such as the cauliflower mosaic virus (CaMV). Promoters from plant viruses are usually
preferred because they are more potent than plant gene promoters, allowing the GM gene to
be expressed at higher levels and hence allowing higher production of the GM protein.
If the gene of interest is not from a plant (for example, if it is from a bacterium or animal),
it is typically modified in other ways as well, to make it more compatible with the gene
expression machinery of the recipient plant cells.
Genetic engineers use a variety of enzymes to cut DNA into specific sequences and to
splice the various pieces of DNA into the plasmid that carries the cloned gene or gene of
interest. The result of many cutting and splicing steps is the complete genetically engineered
construct, called the gene cassette.
For example, the gene of interest in first-generation GM Roundup® Ready soy, maize, cotton
and canola encodes an enzyme (CP4 EPSPS), which confers tolerance to Roundup herbicide.
The CP4 EPSPS gene was isolated from a naturally occurring soil bacterium. In order to
ensure that the CP4 EPSPS gene is switched on appropriately in plants, it is linked to the
CaMV 35S promoter, which is derived from the cauliflower mosaic virus. The CP4 EPSPS
gene is also linked at its leading end to a gene fragment called a signal sequence, obtained
from the petunia, a flowering plant. This is to ensure that the CP4 EPSPS enzyme localizes to
the right place within the plant cells. Finally, a sequence that functions to terminate mRNA
synthesis is spliced to the end of the CP4 EPSPS gene. This termination sequence is taken
from a second bacterial species, Agrobacterium tumefaciens (A. tumefaciens).
Therefore the first-generation Roundup Ready GM tolerance GM gene cassette combines
gene sequences from four diverse organisms: two species of soil bacteria, a flowering plant,
and a plant virus. These all end up in the genetically engineered agricultural crop. This
graphically illustrates the extreme combinations of genetic material that can be brought
about by the GM process. This is something that would never occur naturally.
In addition to the gene(s) that confer traits relevant to the final crop, another gene unit is
often included in the gene cassette along with the gene of interest. This additional gene unit
functions as a selectable marker, meaning that it expresses a function that can be selected
for. Typically this is survival in the presence of an antibiotic or herbicide. The GM gene itself
can be used as a surrogate marker gene if it encodes resistance to a herbicide. When the
marker gene (along with the other gene(s) in the cassette) is successfully engineered into the
GMO Myths and Truths 26