Antisense Technology

Antisense refers to opposing the normal order (“sense”) of the code in DNA. The DNA (deoxyribonucleic acid) in genes directs cells to assemble the proteins which comprise living creatures. The order of bases in DNA corresponds to the ordering of amino acids to form the proteins. To produce protein, the DNA of the genes in cells is first transcribed into a very similar molecule called RNA. RNA can move out of the cell’s nucleus, where the genes have to stay. In the surrounding cytoplasm, proteins are put together according to the RNA’s sequence of bases, matching the DNA instructions.

Antisense molecules prevent the protein assembly machinery from seeing the genetic instructions on how to order the amino acids. If scientists make a molecule that complements the sequence of bases in the RNA, it will stick to the RNA. The antisense molecule, bound to RNA, will prevent the RNA from making protein. Just as two complementary pieces of Velcro stick together, hiding their loops, the antisense molecules bind to RNA and hide its instructions.Thus, antisense stops the synthesis of the protein coded for by the targeted RNA. In effect antisense has turned off the specific gene, or DNA, that was coding for that protein.

Figure 8: Antisense DNA or RNA binds to a specific mRNA and prevents it from being translated into protein.

For example, tomatoes are aware have a small shelf life. Its value as a source of lycopene is well known and therefore there is a need for increasing its shelf life. How does the antisense technology work in delaying ripening of tomatoes? The answer lies in manipulating the three genes responsible for the fruit ripening in tomato-pTOM5 (phytoene synthase, responsible for lycopene synthesis), pTOM6 (polygalacturonase, cell wall degradation) and pTOM13(ACC oxidase, ethylene formation). Antisense technique involves creating a gene construct that will give rise to a RNA transcript complementary to normal or sense RNA. Thus, with the introduction of antisense nucleic acid(DNA or RNA),double stranded RNA hybrids are formed between the antisense RNA and endogenous mRNA which are recognized by the plant’s cell defense mechanism and degraded at the earliest.

In tomatoes, antisense PG (polygalactouranase enzyme) flanked by CaMV-35 promoter has been inserted due to which the functioning of polygalatouranase in cell wall degradation and fruit ripening is delayed, thus increasing the shelf life. In the same way, Antisense ACC oxidase gene responsible for the production of ethylene is silenced due to which the post-harvest spoilage of fruits and vegetables are controlled. The use of this methodology for fruits and vegetables can be a boon for farmers and they can be assured of their harvests having a long shelf life in future.

Figure 9: First genetically engineered tomatoes.