Transcriptional gene silencing in plants: targets, inducers and regulators

Gene silencing can occur either through repression of transcription, term e d transcriptional gene silencing (TGS), or through mRNA degradation,termed post-transcriptional gene silencing (PTGS). Initially, TGS was associated w ith the regulation of transposons through DNA methylation in the nucleus, w hereas PTGS was shown to regulate virus infection through double-stranded R NA in the cytoplasm. However, several breakthroughs in the field have been reported recently that blur this neat distinction. First, in plants TGS and DNA methylation can be induced by either dsRNA or viral infection. Second, a mutation in the plant MOM gene reverses TGS without affecting DNA methyl ation. Third, in Caenorhabditis elegans mutation of several genes that cont rol RNA interference, a form of PTGS, also affect the regulation of transpo sons. TGS and PTGS, therefore, appear to form two alternative pathways to c ontrol incoming, redundant and/or mobile nucleic acids.