Common replication strategies emerging from the study of diverse groups of positive-strand RNA viruses

Studies using brome mosaic virus (BMV), Sindbis virus and poliovirus have provided evidence that disparate groups of plant and animal positive strand RNA viruses have remarkably similar replication strategies. The conservation of several functional domains within virus-encoded nonstructural proteins implies that, although the precise character of these and interacting host components varies for each virus, they employ similar mechanisms for RNA replication. For (+) strand replication, similarities in cis-acting sequence motifs and RNA secondary structures within 5' termini of genomic (+) strands have been identified and have been shown to participate in binding of host factors. The model presented for replication of BMV RNA suggests that binding of these factors to internal control region (ICR) sequence motifs in the double-stranded replication intermediate releases a single-stranded 3' terminus on the (-) strand that may be essential for initiation of genomic (+) strand synthesis. ICR sequences internal to the BMV genome were also found to be required for efficient replication. Asymmetric production of excess genomic (+) over (-) strand RNA, characteristic of all (+) strand viruses, may be accomplished through transition of the replicase from competence for (-) to (+) strand synthesis by the recruitment of additional host factors.