ABSTRACT
The Togaviridae and Flaviviridae are closely related families of enveloped RNA viruses. The defined families of Togaviridae, Flaviviridae, and Bunyaviridae were grouped ecologically as arthropod-borne "arboviruses", with evidence for transmission by blood-sucking in sects. Electron microscopy demonstrated that bunyaviruses comprise a structurally distinct family of arboviruses: symmetry of their nucleocapsids is helical rather than icosohedral. A nucleocapsid C protein is translated on free polysomes in the cytoplasm, and then rapidly unites with 42 S genomic RNA. Completion of nucleocapsid assembly occurs in close proximity to cytomembranes, and with progression of infection, the nucleocapsids accumulate on the cytoplasmic faces of vacuoles derived from the rough endoplasmic reticulum. The assembly of flavivirus nucleocapsids is another major puzzle: unlike alphavirus morphogenesis, there has been no ultrastructural evidence for intermediate accumulation of virion structural precursors. The formation of nucleocapsids occurs very efficiently and rapidly after the translation of C protein.
