Virus-encoded suppressor of posttranscriptional gene silencing targets a maintenance step in the silencing pathway

Certain plant viruses encode suppressors of posttranscriptional gene silenc ing (PTGs), an adaptive antiviral defense response that limits virus replic ation and spread. The tobacco etch potyvirus protein, helper component-prot einase (HC-Pro), suppresses PTGS of silenced transgenes. The effect of HC-P ro on different steps of the silencing pathway was analyzed by using both t ransient Agrobacterium tumefaciens-based delivery and transgenic systems. H C-Pro inactivated PTGS in plants containing a preexisting silenced beta -gl ucuronidase (GUs) transgene. PTGS in this system was associated with both s mall RNA molecules (21-26 nt) corresponding to the 3' proximal region of th e transcribed GUS sequence and cytosine methylation of specific sites near the 3' end of the GUs transgene. Introduction of HC-Pro into these plants r esulted in loss of PTGs, loss of small RNAs, and partial loss of methylatio n. These results suggest that HC-Pro targets a PTGs maintenance (as opposed to an initiation or signaling) component at a point that affects accumulat ion of small RNAs and methylation of genomic DNA.