Analysis of the 3′ cis -Acting Elements of Rubella Virus by Using Replicons Expressing a Puromycin Resistance Gene

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Abstract
A rubella virus (RUB) replicon, RUBrep/PAC, was constructed and used to map the 3′ cis -acting elements (3′ CSE) of the RUB genome required for RUB replication. The RUBrep/PAC replicon had the structural protein open reading frame partially replaced by a puromycin acetyltransferase (PAC) gene. Cells transfected with RUBrep/PAC transcripts expressed the PAC gene from the subgenomic RNA, were rendered resistant to puromycin, and thus survived selection with this drug. The relative survival following puromycin selection of cells transfected with transcripts from RUBrep/PAC constructs with mutations in the 3′ CSE varied. The 3′ region necessary for optimal relative survival consisted of the 3′ 305 nucleotides (nt), a region conserved in RUB defective-interfering RNAs, and thus this region constitutes the 3′ CSE. Within the 3′ CSE, deletions in the ∼245 nt that overlap the 3′ end of the E1 gene resulted in reduced relative survivals, ranging from 20 to <1% of the parental replicon survival level while most mutations within the ∼60-nt 3′ untranslated region (UTR) were lethal. None of the 3′ CSE mutations affected in vitro translation of the nonstructural protein open reading frame (which is 5′ proximal in the genome and encodes the enzymes involved in virus RNA replication). In cells transfected with replicons with 3′ CSE mutations that survived antibiotic selection (i.e., those with mutations in the region of the 3′ CSE that overlaps the E1 coding region), the amount of replicon-specific minus-strand RNA was uniform; however, the accumulation of both plus-strand RNA species, genomic and subgenomic, varied widely, indicating that this region of the RUB 3′ CSE affects plus-strand RNA accumulation rather than minus-strand RNA synthesis.