DNA methylation and chromatin structure affect transcriptional and post-transcriptional transgene silencing in Arabidopsis

In plants, transgenes can be silenced at both the transcriptional [1] and p ost transcripitonal levels [2]. Methylation of the transgene promoter corre lates with transcriptional gene silencing (TGS) [3] whereas methylation of the coding sequence is associated with post-transcriptional gene silencing (PTGS) [4]. In animals, TGS requires methylation and changes in chromatin c onformation [5]. The involvement of methylation during PTGS in plants is un clear and organisms with nonmethylated genomes such as Caenorhabditis elega ns or Drosophila can display RNA interference (RNAi), a silencing process m echanistically related to PTGS [6]. Here, we crossed Arabidopsis mutants im paired in a SWI2/SNF2 chromatin component (ddm1 [7]) or in the major DNA me thyltransferase (met1 [8] and E. Richards, personal communication) with tra nsgenic lines in which a reporter consisting of the cauliflower mosaic viru s 35S promoter fused to the beta -glucuronidase (GUS) gene (35S-GUS) was si lenced by TGS or PTGS. We observed an efficient release of 35S-GUS TGS by b oth the ddm1 and met1 mutations and stochastic release of 35S-GUS PTGS by t hese two mutations during development. These results show that DNA methylat ion and chromatin structure are common regulators of TGS and PTGS.