Glufosinate chemically resembles the amino acid glutamate and acts
to inhibit an enzyme, called glutamine synthetase, which is
involved in the synthesis of glutamine. Essentially, glufosinate
acts enough like glutamate, the molecule used by glutamine
synthetase to make glutamine, that it blocks the enzyme's usual
activity. Glutamine synthetase is also involved in ammonia
detoxification. The presence of glufosinate results in reduced
glutamine levels and a parallel increase in concentrations of
ammonia in plant tissues. This leads to cell membrane disruption
and cessation of photosynthesis resulting in plant withering and
Glufosinate tolerance in this maize line is the result of
introducing the bar gene encoding the enzyme
phosphinothricin-N-acetyltransferase (PAT) isolated from the common
aerobic soil actinomycete, Streptomyces hygroscopicus. The PAT
enzyme catalyzes the acetylation of phosphinothricin, detoxifying
it into an inactive compound. The PAT enzyme is not known to have
any toxic properties.
The male-sterile trait was introduced by inserting the barnase
gene, isolated from Bacillus amyloliquefaciens, a common
soil bacterium that is frequently used as a source for industrial
enzymes. The barnase gene encodes for a ribonuclease enzyme (RNAse)
that is expressed only in the tapetum cells of the pollen sac
during anther development. The RNAse affects RNA production,
disrupting normal cell functioning and arresting early anther
development, thus leading to male sterility.
The introduced DNA also included the barstar gene from B.
amyloliquefaciens that encodes a specific inhibitor of the
barnase RNAse. The barstar gene was included to prevent the RNAse
from disrupting the development of bacteria in which the introduced
DNA was prepared.
Two antibiotic resistant genes were also introduced into this LMO:
the beta-lactamase gene (bla) and chloramphenicol acetyl
transferase (cat) genes, encoding resistance to the antibiotics
ampicillin and chloramphenicol, respectively. These were included
as selectable marker genes to identify bacteria transformed with
recombinant plasmids. The expression of these three genes (barstar,
bla, and cat) was regulated by bacterial promoters that are not
active in plants.