The 1345-4 line of tomato (Lycopersicon esculentum) was developed
through a specific genetic modification of cultivar 91103-114 to
exhibit a decreased activity of the enzyme 1-aminocyclopropane-1-
carboxylic acid (ACC) synthase. 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 in situ accumulation of
ethylene in the transgenic tomatoes is 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.
While not completely understood, the mechanism of "downregulation"
of the endogenous ACC synthase gene is likely linked to the
coordinate suppression of transcription of both the endogenous gene
and the introduced truncated ACC synthase gene.