The structural proteins (SP) of the Togaviridae can be deleted in defective interfering RNAs. The dispensability of viral SP has allowed construction of noninfectious viral expression vectors and replicons from viruses of the Alphavirus and Rubivirus genera. Nevertheless, in this study, we found that the SP of rubella virus (RUB) could enhance expression of reporter genes from RUB replicons in trans. SP enhancement required capsid protein (CP) expression and was not due to RNA-RNA recombination. Accumulation of minus-and plus-strand RNAs from replicons was observed in the presence of SP, suggesting that SP specifically affects RNA synthesis. By using replicons containing an antibiotic resistance gene, we found 2-to 50-fold increases in the number of cells surviving selection in the presence of SP. The increases depended significantly on the amount of transfected RNA. Small amounts of RNA or templates that replicated inefficiently showed more enhancement. The infectivity of infectious RNA was increased by at least 10-fold in cells expressing CP. Moreover, virus infectivity was greatly enhanced in such cells. In other cells that expressed higher levels of CP, RNA replication of replicons was inhibited. Thus, depending on conditions, CP can markedly enhance or inhibit RUB RNA replication.Rubella virus (RUB) is the sole member of Rubivirus genus in the Togaviridae family. RUB has a single-stranded, positivesense RNA genome that is 9,762 nucleotides (nt) in length. The genome encodes two open reading frames (ORFs): the 5Ј-proximal ORF encodes viral nonstructural proteins (NSP) that are responsible for viral genome replication, while the 3Ј-proximal ORF encodes three virion structural proteins (SP), the capsid protein (CP), and the two envelope glycoproteins, E2 and E1.A polyprotein, P200, is translated from the 5Ј-proximal ORF and is a major component of the replication complex for the synthesis of genomic minus-strand RNA, which serves as a template for the synthesis of plus-strand genomic and subgenomic RNA, the mRNA for the virion proteins. The switch of synthesis from minus-strand genomic RNAs to plus-strand RNA synthesis is dependent on the processing of P200 by the virally encoded protease (17), a mechanism similar to that observed in the NSP of Sindbis virus (16). Studies of RUB defective interfering (DI) RNAs have shown that DI RNAs generated during undiluted passaging of virus or persistently infected cells have most of the SP coding region removed, showing that most of the SP coding region is not required for replication of the viral genome (5, 6). RUB replicons have been constructed which have part of the SP coding region replaced by genes encoding the puromycin-resistance protein (PAC) (4) or green fluorescent protein (GFP) (28), respectively. Using the RUBrep/PAC, Chen et al. (4) identified the minimal region at the 3Ј end of the RUB genome that is required for viral genome replication and measured the replication efficiency of various 3Ј constructs. Using RUBrep/GFP, Tzeng et al. (28) showed that most o...