SDE3 encodes an RNA helicase required for posttranscriptional gene silencing in Arabidopsis

post-transcriptional gene silencing (PTGS) provides protection in plants ag ainst virus infection and can suppress expression of transgenes, Arabidopsi s plants carrying mutations at the SDE3 locus are defective in PTGS mediate d by a green fluorescent protein transgene. However, PTGS mediated by tobac co rattle virus (TRV) was not affected by sde3, From these results we concl ude that SDE3, like the previously described RNA polymerase encoded by SDE1 , acts at a stage in the mechanism that is circumvented when PTGS is mediat ed by TRV, The product of SDE3 is similar to RNA helicase-like proteins inc luding GB110 in mouse and other proteins in Drosophila and humans. These pr oteins are similar to, but clearly distinct from Upf1p and SMG-2, which are required for nonsense-mediated mRNA decay in yeast and Caenorhabditis eleg ans and, in the case of SMG-2, for PTGS.