<p style="text-align: justify;">Transgenic cotton lines 31807 and 31808 were developed using recombinant DNA techniques to be both resistant to major caterpillar pests of cotton and to herbicides in the oxynil family, principally bromoxynil (tradename Buctril&reg;). These cotton lines express a modified gene (<i>cry1Ac</i>) that encodes an insecticidal crystalline Cry1Ac delta-endotoxin protein, derived from the soil bacterium <i>Bacillus thuringiensis</i> subsp. <i>kurstaki (B.t.k)</i> strain HD73. Insecticidal activity is caused by the selective binding of Cry1Ac protein to specific sites localized on the brush border midgut epithelium of susceptible insect species. Following binding, cation-specific pores are formed that disrupt midgut ion flow and thereby cause gut paralysis and eventual death due to bacterial sepsis. Delta-endotoxins, such as the Cry1Ac protein expressed in cotton lines 31807 and 31808, exhibit highly selective insecticidal activity against a narrow range of lepidopteran insects such as cotton bollworm, tobacco budworm and pink bollworm. The specificity of action is directly attributable to the presence of specific receptors in the target insects. There are no receptors for delta-endotoxins of <i>B. thuringiensis</i> on the surface of mammalian intestinal cells, therefore, livestock animals and humans are not susceptible to these proteins.</p>

<p style="text-align: justify;">These cotton lines contain a second gene (<i>bxn</i>) that encodes for a bacterial enzyme, nitrilase, from <i>Klebsiella pneumoniae</i>. The nitrilase enzyme hydrolyses the herbicides ioxynil and bromoxynil into non-phytotoxic compounds. Specifically, the nitrilase enzyme breaks down bromoxynil into 3,5 dibromo-4-hydroxybenzoic acid (DBHA), eliminating its herbicidal activity.</p>

<p style="text-align: justify;">An antibiotic resistance marker gene (<i>neo</i>) encoding the enzyme neomycin phosphotransferase II (NPTII), which inactivates aminoglycoside antibiotics such as kanamycin and neomycin, was also introduced into the genome of these transgenic lines. This gene was derived from a bacterial transposon (Tn5 transposable element from <i>Escherichia coli</i>) and was included as a selectable marker to identify transformed plants during tissue culture regeneration and multiplication. The expression of the <i>neo</i> gene in these plants has no agronomic significance and the safety of the NPTII enzyme as a food additive was evaluated by the United States Food and Drug Administration in 1994 (US FDA, 1994).</p>
