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Function and properties of silencing suppressor proteins

RNA interference (RNAi)

RNA interference (RNAi) is a recently identified mechanism that influences expression of genes in a wide variety of organisms, including fungi, nematodes, fruit fly (Drosophila), plants and mammalian cells. RNAi has been implicated in several, different processes including the temporal regulation of developmental gene expression, and the protection of genome integrity by prevention of transposon mobilisation. In addition, in plants, RNAi also functions as a resistance mechanism against virus infection by targeting virus-specific RNA molecules for degradation.

Many, perhaps all, plant viruses encode suppressor proteins that interrupt RNAi and facilitate virus infection. Studies in other laboratories have shown that plant virus silencing suppressor proteins operate in a number of ways, including reversing silencing in tissues in which it is already established, preventing initiation of silencing in newly emerging tissues or preventing spread of a systemic signal that induces silencing in other tissues.

We have discovered recently that some plant virus silencing suppressor proteins are functional when expressed in cultured Drosophila cells. We believe that silencing suppressor proteins can be used as probes to dissect the RNAi pathway. We have initiated a new research programme to identify Drosophila proteins that are targeted by and interact with various plant virus silencing suppressor proteins, and we will use the information obtained from these studies to investigate the RNAi pathways in plants and other higher organisms.

Schematic of the Pathway of RNAi. (Modified from Dykxhoorn, D.M., Novina, C.D. and Sharp, P.A. 2003. Killing the Messenger: Short RNAs that Silence Gene Expression. Nature Revs. Mol. Cell. Biol. 4: 457-467.) Once an mRNA is cleaved by RNAi, since the ends of the mRNA are no longer protected by a 5' cap and a 3' poly(A) tail, the free ends of the two mRNA fragments are rapidly degraded by cell RNases.

The figure depicts our current model on RNAi function which is based on our own data and on results obtained by others in the model systems C. elegans and Drosophila.

A cellular pathway mitigates post-transcriptional gene silencing. Long double-stranded RNA or microRNA hairpin RNAs are processed by the cellular factor Dicer to form siRNAs or miRNAs. These RNAs are used by a RNAi silencing complex to degrade mRNA (RNAi) or to translationally repress the mRNA (siRNA). Little is known about the mechanisms behind small RNA directed silencing, and even less about the factors which participate in these two processes.