RNA TEMPLATE-DIRECTED RNA-SYNTHESIS BY T7 RNA-POLYMERASE

In an attempt to synthesize an oligoribonucleotide by run-off transcri ption by bacteriophage T7 RNA polymerase, a major transcript was produ ced that was much longer than expected. Analysis of the reaction indic ated that the product resulted from initial DNA-directed run-off trans cription followed by RNA template-directed RNA synthesis. This reactio n occurred because the RNA made from the DNA template displayed self-c omplementarity at its 3' end and therefore could form an intra- or int ermolecular primed template. In reactions containing only an RNA templ ate, the rate of incorporation of NTPs was quite comparable to DNA-dep endent transcription. RNA template-directed RNA synthesis has been fou nd to occur with a great number of oligoribonucleotides, even with pri med templates that are only marginally stable. In one instance, we obs erved a multistep extension reaction converting the oligonucleotide in to a final product longer than twice its original length. Presumably, such a process could have generated some of the RNAs found to be effic iently replicated by T7 RNA polymerase.