Transcriptional and posttranscriptional gene silencing are mechanisticallyrelated

Two distinct gene-silencing phenomena are observed in plants: transcription al gene silencing (TGS), which involves decreased RNA synthesis because of promoter methylation, and posttranscriptional gene silencing (PTGS), which involves sequence-specific RNA degradation. PTGS is induced by deliberate [ 1-4] or fortuitous production (R,v,B,, unpublished data) of double-stranded RNA (dsRNA), TGS could be the result of DNA pairing [5], but could also be the result of dsRNA, as was shown by the dsRNA-induced inactivation of a t ransgenic promoter [6]. Here, we show that when targeting flower pigmentati on genes in Petunia, transgenes expressing dsRNA can induce PTGS when codin g sequences are used and TGS when promoter sequences are taken. For both ty pes of silencing, small RNA species are found, which are thought to be dsRN A decay products [7] and determine the sequence specificity of the silencin g process [8, 9]. Furthermore, silencing is accompanied by the methylation of DNA sequences that are homologous to dsRNA, DNA methylation is assumed t o be essential for regulating TGS and important for reinforcing PTGS [10], Therefore, we conclude that TGS and PTGS are mechanistically related. In ad dition, we show that dsRNA-induced TGS provides an efficient tool to genera te gene knockouts, because not only does the TGS of a PTGS-inducing transge ne fully revert the PTGS phenotype, but also an endogenous gene can be tran scriptionally silenced by dsRNA corresponding to its promoter.