Post-transcriptional gene silencing mutants

It has been known for more than a decade that increasing the gene copy numb er does not necessarily lead to increased gene activity. Plants have develo ped efficient mechanisms such as post-transcriptional gene silencing (PTGS) to regulate abnormal gene expression in a sequence-specific fashion. PTGS of (trans)genes can be inhibited by non-homologous viruses, and PTGS-impair ed mutants can be hypersensitive to such viruses, indicating that in plants this mechanism is triggered to protect against viral invasion. Genetic ana lysis of a related phenomenon, quelling, in Neurospora has led to the ident ification of two genes encoding proteins that share homologies with RNA-dep endent RNA polymerases and with DNA helicases. This finding reinforces prev ious models in which PTGS involves RNA molecules complementary to the RNA s pecies targeted for degradation. Insight into the mechanisms of PTGS may al so be obtained in other distant organisms such as Caenorhabditis elegans in which a related phenomenon, RNA interference, has been genetically studied , leading to the identification of two genes encoding proteins sharing homo logies with a translation factor and an RNase D.