the present invention relates to the field of synthesis of short double - stranded rnas . an in vitro transcription method using bacteriophage polymerases and target sequence - specific single - stranded dna oligonucleotides as templates is disclosed . the present invention finds particularly advantageous use in the synthesis of short interfering rnas that have been shown to function as key intermediates in triggering sequence - specific rna degradation during posttranscriptional gene silencing in plants and rna interference in invertebrates and vertebrate systems .