<p style="text-align: justify;">Tomato line 1345-4 was developed using recombinant DNA techniques to express the trait of delayed ripening of tomato fruit. The transgenic line contains a truncated version of the tomato 1-aminocyclopropane-1-carboxyllic acid (ACC) synthase gene, normally found in tomato. This endogenous enzyme is responsible for the conversion of s-adenosylmethionine to ACC, which is the immediate precursor of ethylene, a phytohormone known to play a key role in fruit ripening. The presence of the truncated ACC synthase gene suppresses the normal expression of the native ACC synthase gene, and while not completely understood, the mechanism of &ldquo;downregulation&rdquo; is likely linked to the coordinate suppression of transcription of both the endogenous gene and the introduced truncated ACC synthase gene. The <i>in situ</i> accumulation of ethylene in the transgenic tomatoes was only about 1/50 the level found in the unmodified parental line and the fruit does not fully ripen unless an external source of ethylene is applied. 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 this transgenic tomato. 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>
