ARABIDOPSIS MUTANTS IMPAIRED IN COSUPPRESSION

Post-transcriptional gene silencing (cosuppression) results in the deg radation of RNA after transcription. A transgenic Arabidopsis line sho wing post-transcriptional silencing of a 35S-uidA transgene and uidA-s pecific methylation was mutagenized using ethyl methanesulfonate. Six independent plants were isolated in which uidA mRNA accumulation and b eta-glucuronidase activity were increased up to 3500-fold, whereas the transcription rate of the 35S-uidA transgene was increased only up to threefold. These plants each carried a recessive monogenic mutation t hat is responsible for the release of silencing. These mutations defin ed two genetic loci, called sgs1 and sgs2 (for Suppressor of gene Sile ncing). Transgene methylation was distinctly modified in sgs1 and sgs2 mutants. However, methylation of centromeric repeats was not affected , indicating that sgs mutants differ from ddm (for decrease in DNA met hylation) and som (for somniferous) mutants. Indeed, unlike ddm and so m mutations, sgs mutations were not able to release transcriptional si lencing of a 35S-hpt transgene. Conversely, both sgs1 and sgs2 mutatio ns were able to release cosuppression of host Nia genes and 35S-Nia2 t ransgenes. These results therefore indicate that sgs mutations act in trans to impede specifically transgene-induced post-transcriptional ge ne silencing.