Provided are isolated nucleic acid molecules which comprise an sde3 nucleotide sequence which encodes an SDE3 polypeptide capable of mediating Post Transcriptional Gene Silencing (PTGS) of a transgene in a plant into which the nucleic acid is introduced. The SDE3 polypeptide is believed to encode an RNA helicase-like polypeptide. The invention has utility in both enhancing and suppressing the PTGS of transgenes in plants, and methods and materials are provided to achieve this.
