Cellular expression of a functional nodavirus RNA replicon from vaccinia virus vectors.

RNA replication provides a powerful means for the amplification of RNA, but to date it has been found to occur naturally only among RNA viruses. In an attempt to harness this process for the amplification of heterologous mRNAs, both an RNA replicase and its corresponding RNA templates have been expressed in functional form, using vaccinia virus-bacteriophage T7 RNA polymerase vectors. Plasmids were constructed which contained in 5'-to-3' order (i) a bacteriophage T7 promoter; (ii) a full-length cDNA encoding either the RNA replicase (RNA 1) or the coat protein (RNA 2) of flock house virus (FHV), (iii) a cDNA sequence that encoded the self-cleaving ribozyme of satellite tobacco ringspot virus, and (iv) a T7 transcriptional terminator. Both in vitro and in vivo, circular plasmids of this structure were transcribed by T7 RNA polymerase to produce RNAs with sizes that closely resembled those of the two authentic FHV genomic RNAs, RNA 1 and RNA 2. In baby hamster kidney cells that expressed authentic FHV RNA replicase, the RNA 2 (coat protein) transcripts were accurately replicated. Moreover, the RNA 1 (replicase) transcripts directed the synthesis of an enzyme that could replicate not only authentic virion-derived FHV RNA but also the plasmid-derived transcripts themselves. Under the latter conditions, replicative amplification of the RNA transcripts ensued and resulted in a high rate of synthesis of the encoded proteins. This successful expression from a DNA vector of the complex biological process of RNA replication will greatly facilitate studies of its mechanism and is a major step towards the goal of harnessing RNA replication for mRNA amplification.