Mechanism
Endogenous ta-siRNAs act via hetero-silencing, which means that the genes they target for cleavage and repression do not have much resemblance to the genes from which the siRNAs derive. This differs from other endogenous siRNAs which are cis-acting and perform auto-silencing, repressing the expression of genes that are the same as or have a lot of resemblance to the genes from which they derive. It was previously thought that only miRNAs exhibited hetero-silencing. Like other siRNAs, the ta-siRNAs are incorporated into RNA-induced silencing complexes (RISCs), where they guide the complex to cleave the target mRNAs in the middle of a single complementary site and repress translation.
A member of the Argonaute protein family is a component of all RNA silencing effector complexes, including the RISCs that catalyze mRNA cleavage. Specifically in arabidopsis, it appears to be AGO7/ZIPPY that plays a role in the ta-siRNA pathway by acting during TAS3-derived ta-siRNA-mediated regulation. AGO7/ZIPPY associates with TAS3 ta-siRNAs and cleaves their targets. AGO7/ZIPPY does not seem to play a role in the mechanisms for TAS1 or TAS2 ta-siRNAs, suggesting that the different families have slight changes in their mechanisms in arabidopsis. In addition to AGO7 involvement, ta-siRNAs can be loaded into AGO1 complexes in Arabidopsis to guide target mRNA cleavage.
Read more about this topic: Trans-acting Si RNA
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