ABSTRACT
The alphaviruses and flavivimses are two important groups of positive strand animal viruses which contain a number of human and veterinary pathogens. Members of each group have evolved from common ancestors in response to different environmental factors. During the alphavirus replication cycle, three distinct RNA synthetic activities are needed early in infection, a replicase is needed to synthesize a full-length minus strand template from the genomic RNA. A number of features of the flavivirus genome suggest that flaviviruses are only distantly related to other groups of RNA animal viruses and that they deserve their recent reclassification as a separate family, Flaviviridae. Like the alphaviruses, the flaviviruses have virtually randomized the codon usage even within domains of high amino acid conservation. The intracellular events occurring during flavivirus infection are difficult to study, in part because host macromolecular synthesis is not shut off in infected cells, and in part because flaviviruses in general replicate to low titer.
