<p style="text-align: justify;">The tomato lines B, Da and F were developed using recombinant DNA techniques to display the trait of delayed ripening of tomato fruit. These transgenic tomatoes lines contain a partial polygalacturonase (PG) gene that encodes for the PG protein, a pectin degrading enzyme derived from tomato. The lines differ slightly in that Da and F contain the partial PG gene in the sense orientation while line B contains a partial antisense PG gene, essentially a reverse copy. The presence of the partial PG gene, in either sense or antisense orientation, suppresses the expression of endogenous PG enzyme at the onset of fruit ripening.</p>

<p style="text-align: justify;">In the case of line B, the mechanism of action is likely linked to the hybridization of antisense and sense mRNA transcripts, resulting in a decreased amount of free positive sense mRNA available for protein translation. For lines Da and F, which contain the truncated PG gene in the sense orientation, reduced PG expression may be due to coordinate suppression of transcription of both the endogenous gene and the introduced truncated gene.</p>

<p style="text-align: justify;">In any event, reduced PG expression decreases the breakdown of pectin and leads to fruit with slowed cell wall breakdown, better viscosity characteristics and delayed softening. Tomato lines B, Da and F have improved harvest and processing properties that allow the transgenic tomatoes to remain longer on the vine to develop their natural flavour, maintain their firmness for shipping and produce a thicker consistency in processing.</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 tomatoes. 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>
