Requirement of 3'-terminal guanosine in (-)-stranded RNA for in vitro replication of cucumber mosaic virus satellite RNA by viral RNA-dependent RNA polymerase.

The 3' terminus of the (-) RNA strand in the replicative forms of several (+)-stranded RNA viruses possesses an unpaired guanosine with unknown function. This unpaired guanosine is also found at the 3' terminus of the (-) strand in the double-stranded form of two cucumoviral satellite RNAs. Using a cucumber mosaic virus (CMV) RNA-dependent RNA polymerase capable of replicating the satellite RNA in vitro, the 3'-terminal guanosine of the satellite (-) strand was shown to be an absolute requirement for satellite (+) strand synthesis. If genomic RNA synthesis of CMV and other members of the alphavirus-like superfamily that produce (-) strands terminating in an unpaired 3' guanosine follows a similar strategy, the work reported here would represent the first experimental support for the notion of 3'-terminal guanosine functioning as an essential recognition signal for viral replicases, enabling (+) strand RNA synthesis to be initiated internally from a (-) strand template.