ABSTRACT
RNA viruses are unique in having their information encoded in RNA. Their diversity, their widespread occurrence, and host range reflect their evolutionary success. The RNA in positive-strand viruses has several roles: it acts as messenger for expression of the viral proteins, as template in RNA replication and as core in the assembly of virion particles. The mechanism of viral RNA replication is complicated in vivo by the fact that the viral RNA strand must also be expressed to provide replicase and other gene products, and is eventually packaged. Qß replicase is quite specific in its choice of templates: in vivo, it only accepts Qß RNA itself and its complementary strand. In vitro, other templates have also been found to direct synthesis of their complementary strands (transcription) by Qß replicase; however, they can not replicate autocatalytically. Kinetic studies showed that the template specificity of Qß replicase is controlled kinetically by the lifetimes of the initiation complexes.
