Gene Cloning

376 Gene Cloning
placed on the market: the “Flavr-Savr” tomato produced by Calgene in the
USA. Subsequently, tomato paste made from transgenic tomatoes modified
using antisense technology was sold in supermarkets in the UK.
Although the precise details of how these tomatoes were made differ, the
same central principle was used in each case. A key enzyme called polygalacturonase
is important in the ripening process of tomatoes and other
fruit, causing depolymerization of the pectin present in cell walls and
hence leading to softening. Eventually, this leads to fruit becoming very
soft (over-ripe) and it was this process that was delayed in both types of
transgenic tomatoes. By expressing an antisense version of the gene for
polygalacturonase, the amount of mRNA available for translation into
active enzyme was markedly reduced and this was effective at delaying
softening, although other parts of the ripening process (such as color
change from green to red) took place as normal (Figure 12.8).
The Flavr-Savr tomato was a commercial failure, and the tomato paste
(although successful) was pulled from supermarket shelves following a
strong wave of anti-GM food feeling that swept the United Kingdom in
2000. Whether the failure of Flavr-Savr was due to poor marketing or a poor
product is disputed, but the antisense approach clearly does reduce fruit
softening and in plants has potential in other areas too such as producing
viral resistance.
Q12.5. Can you think of how antisense technology could be used to
reduce the expression of an endogenous gene in selected cell types only,
rather than in the whole organism?
Analysis of gene expression using reporter genes
Much effort in research is put into understanding the signals that turn
genes on and off. The classic example of gene regulation, the study of
which has been inflicted upon many generations of undergraduates, is the
lac operon of E. coli . Regulation of gene expression in eukaryotes is a more
involved and complex process, and is harder to study, than in bacterial systems
where the powerful techniques of bacterial genetics can be applied.
Indeed, until the advent of the use of transgenic organisms, information on
eukaryotic promoters was sparse. But enormous progress has now been
made in this field, and the use of transgenic organisms lies at the heart of
this. In particular, the use of so-called promoter probe vectors has been the
key to helping unravel the intricacies of eukaryotic transcription. These
were described in detail in Section 11.3.
How can transgenic organisms be used with promoter probe technology
to study promoters which show tissue specificity? This has been done with
numerous organisms, promoters and reporter genes. One example is using
the reporter gene GFP to detect muscle-specific expression under the