RNA AS A TARGET AND AN INITIATOR OF POSTTRANSCRIPTIONAL GENE SILENCING IN TRANSGENIC PLANTS

Post-transcriptional gene silencing in transgenic plants is the manife station of a mechanism that suppresses RNA accumulation in a sequence- specific manner. The target RNA species may be the products of transge nes, endogenous plant genes or viral RNAs. For an RNA to be a target i t is necessary only that it has sequence homology to the sense RNA pro duct of the transgene. There are three current hypotheses to account f or the mechanism of post transcriptional gene silencing. These models all require production of an antisense RNA of the RNA targets to accou nt for the specificity of the mechanism. There could be either direct transcription of the antisense RNA from the transgene, antisense RNA p roduced in response to over expression of the transgene or antisense R NA produced in response to the production of an aberrant sense RNA pro duct of the transgene. To determine which of these models is correct i t will be necessary to find out whether transgene methylation, which i s frequently associated with the potential of transgenes to confer pos t-transcriptional gene silencing, is a cause or a consequence of the p rocess.