Plant viral suppressors of post-transcriptional silencing do not suppress transcriptional silencing

Homology-dependent gene silencing is a regulatory mechanism that limits RNA accumulation from affected loci either by suppression of transcription (tr anscriptional gene silencing, TGS) or by activation of a sequence-specific RNA degradation process (post-transcriptional gene silencing, PTGS). The P1 /HC-Pro sequence of plant potyviruses and the 2b gene of the cucumber mosai c virus have been shown to interfere with PTGS. The ability of these viral suppressors of PTGS to interfere with TGS was tested using the 271 locus wh ich imposes TGS on transgenes under 35S or 19S promoters and PTGS on the en dogenous nitrite reductase gene (Nii). Both P1/HC-Pro and 2b reversed PTGS of Nii genes in 271-containing tobacco plants, but failed to reverse TGS of 35S-GUS transgenes in the same plant. P1/HC-Pro expression from a transgen e also failed to suppress either the initiation or maintenance of TGS impos ed by the NOSpro-silencing locus, H-2. These results indicate that PTGS and TGS operate through unlinked pathways or that P1/HC-Pro and 2b interfere a t step(s) in PTGS that are downstream of any common components in the two p athways. The data suggest a simple assay to identify post-transcriptionally silenced transgenic lines with the potential to be stably converted to hig h expressing lines.