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 h ave provided evidence that disparate groups of plant and animal positi ve strand RNA viruses have remarkably similar replication strategies. The conservation of several functional domains within virus-encoded no nstructural proteins implies that, although the precise character of t hese and interacting host components varies for each virus, they emplo y similar mechanisms for RNA replication. For (+) strand replication, similarities in cis-acting sequence motifs and RNA secondary structure s within 5' termini of genomic (+) strands have been identified and ha ve been shown to participate in binding of host factors. The model pre sented for replication of BMV RNA suggests that binding of these facto rs to internal control region (ICR) sequence motifs in the double-stra nded replication intermediate releases a single-stranded 3' terminus o n the (-) strand that may be essential for initiation of genomic (+) s trand synthesis. ICR sequences internal to the BMV genome were also fo und to be required for efficient replication. Asymmetric production of excess genomic (+) over (-) strand RNA, characteristic of all (+) str and viruses, may be accomplished through transition of the replicase f rom competence for (-) to (+) strand synthesis by the recruitment of a dditional host factors.