Rubella virus DI RNAs and replicons: requirement for nonstructural proteins acting in cis for amplification by helper virus.

A rubella virus (RUB) replicon was constructed by replacing the 3' proximal structural protein ORF (SP-ORF) in Robo402, a RUB infectious cDNA clone, with a reporter gene, green fluorescent protein (GFP). This replicon, RUBrep/GFP, mimics naturally occurring RUB defective-interfering (DI) RNAs generated during serial undiluted passage that maintain the 5' proximal nonstructural protein ORF (NS-ORF) but contain deletions in the SP-ORF. Following transfection of Vero cells with in vitro RNA transcripts from RUBrep/GFP, replicon replication occurred and the replicon was amplified and spread to other cells in the presence of standard helper virus. GFP expression was a much more sensitive indicator of replicon replication than was Northern analysis to detect replicon-specific RNAs. Most of a series of RUBrep/GFP constructs with deletions in the NS-ORF not only were incapable of self-replication, but were not amplified by standard helper virus. The only exception was a construct with an in-frame deletion between two NotI sites that removed nucleotides 1685-2192 of the genome; this construct did not express GFP by itself, but did express GFP in the presence of standard helper RUB and was spread to other cells. Thus, with the exception of this region, the NS-ORF is required in cis for amplification of RUB replicons by standard helper virus, explaining the selection of DI RNAs that maintain the NS-ORF. Surprisingly, when the NotI deletion was introduced into Robo402, a viable virus resulted that replicated only threefold less efficiently than did Robo402 virus. Thus, the NotI region of the NS-ORF is not necessary for virus replication. This deletion covers a region of the NS-ORF without predicted function, which therefore may function as a spacer or hinge between functional domains. Nevertheless, it was an unexpected finding that a small virus such as RUB could dispense with approximately 10% of its genome.