Richard Jorgensen and his colleagues introduced chalcone synthase, a gene that produces pigments, into petunia under the control of a very strong promoter. But instead of getting purple coloured plants as expected, the flowers were instead white and variegated. They named this cosuppression as two genes were suppressed at the same time, the introduced gene and the homologous endogenous gene in the petunia. This happened in 1990 and in 2002, this phenomena now known as gene silencing, was awarded the discovery of the year award by Science magazine.
It has been shown since that this phenomenon occurs in many plants, animals an example being the nematode Caenorhabditis elegans where it was referred as RNA interference and in fungi where it was first observed in the bread mold Neurospora crassa, and was called quelling. Gene silencing has been shown to have roles in viral defence and transposon silencing mechanisms and thus can be used as a tool in engineering virus resistance or antiviral responses in organisms. It can be done at post transcriptional or at transcriptional level.
Post transcriptional gene silencing (PTGS) is whereby the steady state of messenger RNA (mRNA) is reduced by degradation of the transcribed, fully processed mRNA without changing the transcription rate of the target gene. This degradation is sequence specific. The actual silencing occurs when an endogenous gene is silenced by either introducing a homologous double stranded RNA (dsRNA), where its referred as RNA interference (RNAi), introducing a transgene as was the case in N. crassa where it was referred as quelling or by introducing a virus, mainly done in plants where its referred as virus-induced gene silencing (VIGS).
When dsRNA is introduced into an organism, mRNA is degraded as already stated. The steps are as follows:
Introduced dsRNA is cleaved into short 21-23 nucleotide small interfering RNAs (siRNAs) by the ribonuclease Dicer, inside the cell.
The siRNAs then assemble subsequently with protein components into an RNA-induced silencing complex (RISC) and the RISC complex is activated by ATP-generated unwinding of the siRNAs. The active RISC then binds to the homologous transcript by base pairing interactions and Slicer cleaves the target mRNA. This sequence specific degradation of mRNA results in the gene being silenced.
As stated above gene silencing can also occur at transcription level, referred to as transcriptional gene silencing (TGS). This has been reported to occur by a mechanism of siRNA-mediated DNA methylation, but the mechanism in not yet well understood.
Gene silencing biological roles include protection against viruses infecting plants, producing virus resistant plants, protection against endogenous transposable elements and protection against foreign or duplicated genes
The above mentioned biological roles leads to this technique being used in different applications of biotechnology which include engineering of virus resistant plants when VIGS are introduced, pest control in plants when introduced homologue in the plant can kill off pests by producing dsRNA complementary to essential genes in the pest, leading to these genes being silenced resulting on the pest dying. In animals applications include parasite treatment in mammals which follows the same principle as pest control in plants and gene therapy in humans though this is controversial in modern medicine as exogenous nucleic acids are used, which might not be ideal. In scientific research, gene silencing is being used to knock out specific genes whose sequences are known in order to study their functions, a field known as functional genomics.
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