APPLICATIONS
OF RNA TECHNOLOGY IN TRANSGENICS
As discussed earlier,
transgenic technology at the DNA level allows us to add genes and to inactivate
gene expression by insertion. It is also possible to manipulate genes at the
RNA level by the use of antisense,
ribozymes, or RNA interference.
We have already discussed these topics; therefore, this section simply
summarizes their application in transgenics.
It is possible to insert an
antisense transgene when constructing a transgenic organism. The simplest way
to construct an antisense gene is to invert the DNA sequence of the original
gene. In higher organisms, the cDNA version of the gene is used as the starting
point to avoid complications due to intervening sequences. Such an antisense
gene will be transcribed to give antisense RNA, which will then bind to the
mRNA of the corresponding sense gene. This results in inhibition of gene
expression at the level of translation.
Insertion of antisense genes
into mice has resulted in up to 95% inhibition of gene expression, although the
effectiveness varies greatly. The antisense RNA may be full length or just a
short segment corresponding to part of the sense gene. Short segments
complementary to the 5′-untranslated region of the
sense mRNA are often very effective at blocking translation. Obviously,
antisense constructs can be placed under the control of inducible promoters.
This allows antisense
silencing of gene expression to be regulated by the experimenter.
It is also possible to insert
transgenic constructs that express ribozymes. In such cases, the ribozyme is
usually designed to cleave a specific mRNA. When the ribozyme is expressed it
cleaves the mRNA of the target gene and so reduces gene expression. Occasional
cases are known in both Drosophila
and mice where transgenic expression of ribozymes has been successful in
decreasing gene expression.
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