Pyridoxal-dependent decarboxylases that act on ornithine-, lysine-, arginine- and related substrates can be classified into different familieson the basis of sequence similarity [<cite idref="PUB00003632"/>, <cite idref="PUB00001452"/>]. One of these families includeseukaryotic ornithine decarboxylase (ODC), which catalyses the transformation of ornithine into putrescine; prokaryotic diaminopimelic acid decarboxylase(DAPDC), which catalyses the conversion of diaminopimelic acid into lysine,the final step of lysine biosynthesis; <taxon tax_id="322">Pseudomonas syringae pv. tabaci</taxon>protein, tabA, which is probably involved in tabtoxin biosynthesis andis similar to DAPDC; and bacterial and plant biosynthetic argininedecarboxylase (ADC), which catalyses the transformation of arginine into agmatine, the first step in putrescine synthesis from arginine.<p>Although these proteins, which are known collectively as group IVdecarboxylases [<cite idref="PUB00001452"/>], probably share a common evolutionary origin, theirlevels of sequence similarity are low, being confined to a few shortconserved regions.The tomato ADC gene contains an open reading frame encoding a polypeptideof 502 amino acids and a predicted molecular mass of ~55kDa [<cite idref="PUB00006119"/>]. The predicted amino acid sequence shares 47 and 38% identify with oat and <taxon tax_id="562">Escherichia coli</taxon> ADCs, respectively. Gel blot hybridisation experiments show that,in tomato, ADC is encoded by a single gene and is expressed as a transcriptof ~2.2 kb in the fruit pericarp and leaf tissues [<cite idref="PUB00006119"/>].</p>