Viral suppressors of RNAi

Virale suppressors of RNAi

RNAi is a complex mechanism (Fig. 1) that in plants, on one hand plays an important role in the regulation of endogenes and that on the other hand mainly serves to control the expression of foreign nuclei acids. Apart from inactivating genome integrated elements, e. g. transposons and retroviruses, RNAi functions as a defence mechanism against viruses (Fig. 1, light blue box). However, during evolution numerous plant viruses have developed silencing suppressor proteins that interfere with RNAi. Most of these proteins bind 21 to 24 nucleotide long double stranded RNA (dsRNA) molecules which have a key function in RNAi (Fig.1). Since endogenous small dsRNAs (microRNAs (miRNAs) and trans-acting short interfering RNAs (tasiRNAs)) are essential for normal plant development the lack of such RNA molecules has tremendous effects. In particular, plants in which miRNA-mediated gene regulation is affected display massive phenotypic alterations resembling the symptoms caused by virus infections.
In the frame of a DFG project (see Funding) we are currently analysing one of the viral dsRNA-binding suppressor proteins. Using mutagenized variants we will examine the dsRNA binding capacity of these protein. Protein variants were isolated that, when expressed in plants, maintained their silencing suppressor activity but did not cause conspicuous developmental defects. It is reasonable to assume that the altered proteins still interfere with the function of short interfering RNAs (siRNAs) but lost their capacity to hamper miRNA-mediated gene regulation. Further details about the DFG project and the AlPlanta contribution are available at AlPlanta-DFG.

Figure 1: Simplified model of RNAi affecting transgene over-expression and serving as a defence mechanism against viruses.

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