Two types of HDGS have been categorised: 1. Transcriptional Gene Silencing (TGS) 2. Post-Transcriptional Gene Silencing (PTGS).
Type # 1. Transcriptional Gene Silencing (TGS):
Transcriptional gene silencing (TGS) is a gene silencing mechanism in which inactivation of (trans) gene specific RNA synthesis takes place. These are predominently observed in transgenic plants containing multiple copies of homologous transgene or endogenous gene.
TGS is characterized by the sequence specific DNA methylation at promoter region. TGS is believed to occur through direct DNA interaction between silencer transgene locus and other loci exhibit homologous sequence in their promoter region.
In transgenic plants, it has been demonstrated that denova methylation of the transgene promoter sequence results in transcriptional inactivation. The pairing of DNA-DNA probably results in methylation and inturn occurs gene silencing.
Although methylation at promoter region is significance of TGS, it is however, presumed that methylation do not alone suppress transcription. DNA methylation probably induces the transcriptional silencing through chromatin components.
The link between DNA methylation and transcriptional inactivation is aided by repressive protein MeCP2 which specifically binds to transgene region. Some proteins of the repressive complex initiates denova DNA methylation.
Increase in methylation is followed by acquiring condensed chromatin structure by transcriptionally silent transgene. This type of chromatin remodeling is believed to be responsible for maintenance of repressive status and in the propagation of non-symmetrical methylation pattern in plants.
Type # 2. Post-Transcriptional Gene Silencing (PTGS):
PTGS is highly specific and involved several processes such as co-suppression and antisense suppression. These two processes are otherwise called as RNA mediated virus resistance (RMVR). These amazing processes could silence genes or viruses when a high degree of sequence homology between transgene and endogene or viruses. PTGS is mainly concerned with RNA degradation.
These phenomenons were discovered using transgene in transformed plants in which transgene mRNA is able to silence same sense endogenous transcripts and viral genomic RNA. In PTGS, RNA degradation is initiated by endonucleases and then further degraded by exonucleases. It is however remember that not all transgene induce PTGS because PTGS usually occurs in only a small proportion of the plants transformed with the same gene construct.
It is facinating to see the selection of only specific transgene mRNA, among thousands of other endogenous mRNA for degradation. Several models have been proposed regarding enigma of RNA degradation. It was proposed that concentration of particular RNA produced by the transgene(s) and endogenous gene (when present) above certain threshold concentration in the cell and if it is template for plants RNA dependent RNA polymerase (RDRP). Which are then target for degradation by various forces operations within cells?
The features of transgene mRNA, so called abberant mRNA would be distinct from a normal mRNA is due to lack of introns, un-translatability, transcriptional truncation or extension. Atleast two models explain PTGS one is by single copy transgene that are highly expressed.
According to the second model PTGS is associated with low-profile expression of transgene which are inversely repeated and methylated. These conditions are favourable for the production of aberrant double strand RNA (dsRNA), which is active in inducing PTGS.