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Antisense Gene Approach -

In any gene, the DNA strand oriented as 3’-->5' in relation to its promoter is transcribed; this strand is called the antisense strand. The mRNA base sequence, therefore, is complementary to that of the antisense strand. The remaining DNA strand of the gene, called sense strand, is naturally complementary to the antisense strand of the gene.

Therefore, the base sequence of sense strand of a gene is the same as that of the mRNA produced by it. Hence the mRNA produced by a gene in normal orientation is also known as sense RNA.

An antisense gene is produced by inverting, i.e., reversing the orientation of the protein encoding region of a gene in relation to its promoter. As a result, the natural sense strand of the gene becomes oriented in the 3'--> 5' direction with reference to its promoter, and is transcribed. (The normal antisense strand is not transcribed since now its orientation is 5' --> 3'.)

The RNA produced by this gene has the same sequence as the antisense strand of the normal gene (except for T in DNA in the place of U in RNA), and is therefore known as antisense RNA. When an antisense gene is present in the same nucleus as the normal endogenous gene, transcription of the two genes yields antisense and sense RNA transcripts, respectively.

Since the sense and the antisense RNAs are complementary to each other they would pair to produce double-stranded RNA molecules. This event makes

(l) the mRNA unavailable for translation. At the same time,
(2) the RNA double-strand is attacked and degraded by double-stranded RNA specific RNases. Finally,
(3) these events may somehow lead to the methylation of the promoter and coding regions of the normal gene resulting in silencing of the endogenous gene.

The application of antisense RNA technology is explained using the example slow ripening tomato. In tomato, enzyme polygalacturonase (PG) degrades pectin which is the major component of fruit cell wall. This leads to the softening of fruits and a deterioration in fruit quality.

Transgenic tomatoes have been produced which contain antisense construct of the gene encoding PG. These transgenics show a drastically reduced expression of PG and markedly slower ripening and fruit softening. This has greatly improved the shelf-life and the general quality of tomato fruits. Such tomatoes are being marketed in U.S.A. under the name 'FlavrSavr'.

 
 

 
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