<p style="text-align: justify;">Tobacco line Vector 21-41 was developed to reduce the presence of nicotine in the leaves of commercial tobacco (<i>Nicotiania tabacum</i>). Vector 21-41 delivers nicotine levels around 20 times less than conventional tobacco. Nicotine is commonly regarded as the addictive substance in tobacco and is a precursor of Tobacco Specific Nitrosamines which are mutagenic and carcinogenic substances normally found in tobacco. It is thought that delivering nicotine at levels well below those determined to be addictive can reduce dependency on tobacco products.</p>

<p style="text-align: justify;">The synthesis of nicotine is isolated in the roots of tobacco and subsequently transported via the phloem to the leaves. Nicotine is the product of two enzymatic pathways: the methylpyrroline pathway, which produces the nicotine precursor N-methylpyrrolinium cation; and the pyridine nucleotide cycle, which produces the precursor nicotine acid.</p>

<p style="text-align: justify;">In the methylpyrroline pathway, L-ornithine is converted via ornithine decarboxylase to putrescine which in turn is acted on by putrescine N-methyltransferase (PMTase) to produce 4-methyle putrescine and subsequently N-methylpyrrolinium cation. PMTase is the limiting enzyme in this pathway.</p>

<p style="text-align: justify;">The pyridine nucleotide cycle begins with 3-phosphoglyceraldehyde which condenses with aspartic acid, to form quinolinic acid. Quinolinic acid is subject to the action of quinolinic acid phosphoribosyltransferase (QPTase) and through a series of steps becomes nicotinic acid. In this pathway, QPTase is the limiting enzyme.</p>

<p style="text-align: justify;">In Vector 21-41 the pyridine nucleotide cycle was down-regulated through the insertion of the antisense configuration of the gene coding for QPTase (NtQTP1). The reduction in expression of QPTase resulted in a dramatic decrease in the production of nicotinic acid and thus nicotine.</p>
