Assembly of functional Sindbis virus RNA replication complexes: requirement for coexpression of P123 and P34.

A vaccinia virus transient expression system was used to determine which of the Sindbis virus (SIN) proteins and/or polyproteins are necessary for the formation of active replication complexes and, in particular, to analyze the role of nsP4, the putative polymerase, versus P34 in RNA replication. We generated vaccinia virus recombinants in which the cDNA for the entire SIN nonstructural coding region as well as cDNA copies of the individual nonstructural proteins (nsPs) and several intermediate polyproteins were placed downstream of the promoter for T7 RNA polymerase and the encephalomyocarditis virus 5' untranslated region. The proteins expressed by the vaccinia virus recombinants comigrate with authentic proteins synthesized in SIN-infected cells, and the polyproteins appear to be processed to the individual proteins of the correct size. To examine the replication efficiencies of different protein combinations, a vaccinia virus recombinant was designed to express an engineered substrate RNA which could serve as a template for replication and subgenomic mRNA transcription by the SIN nsPs. Expression of the entire SIN nonstructural coding region resulted in the synthesis of high levels of both genomic and subgenomic RNAs derived from the engineered template. No RNA replication could be detected during coexpression of the four individual nsPs, although the proteins were indistinguishable, in terms of electrophoretic mobility, from those synthesized in SIN-infected cells. Coexpression of polyproteins P12, P23, and/or P34 with the individual nsPs also did not result in detectable levels of RNA replication. However, when P123 and P34 were coexpressed, efficient RNA replication and subgenomic mRNA transcription of the substrate RNA was observed. Coexpression of nsP4 with P123 resulted in the synthesis of only minus-strand RNAs. These studies show that expression of both P123 and P34 is necessary for establishment of functional RNA replication and transcription complexes and raise the possibility that the polyproteins themselves may be functional components of these complexes. In addition, these data indicate that an nsP4 moiety expressed independently with an additional N-terminal methionine is capable of functioning in minus-strand but not plus-strand RNA synthesis.