SUBGENOMIC MESSENGER-RNA OF AURA-ALPHAVIRUS IS PACKAGED INTO VIRIONS

Purified virions of Aura virus, a South American alphavirus related to Sindbis virus, were found to contain two RNA species, one of 12 kb an d the other of 4.2 kb. Northern (RNA) blot analysis, primer extension analysis, and limited sequencing showed that the 12-kb RNA was the vir al genomic RNA, whereas the 4.2-kb RNA present in virus preparations w as identical to the 26S subgenomic RNA present in infected cells. The subgenomic RNA is the messenger for translation of the viral structura l proteins, and its synthesis is absolutely required for replication o f the virus. Although 26S RNA is present in the cytosol of all cells i nfected by alphaviruses, this is the first report of incorporation of the subgenomic RNA into alphavirus particles. Packaging of the Aura vi rus subgenomic mRNA occurred following infection of mosquito (Aedes al bopictus C6/36), hamster (BHK-21), or monkey (Vero) cells. Quantitatio n of the amounts of genomic and subgenomic RNA both in virions and in infected cells showed that the ratio of genomic to subgenomic RNA was 3- to 10-fold higher in Aura virions than in infected cells. Thus, alt hough the subgenomic RNA is packaged efficiently, the genomic RNA has a selective advantage during packaging. In contrast, in parallel exper iments with Sindbis virus, packaging of subgenomic RNA was not detecta ble. We also found that subgenomic RNA was present in about threefold- greater amounts relative to genomic RNA in cells infected by Aura viru s than in cells infected by Sindbis virus. Packaging of the Aura virus subgenomic RNA, but not those of other alphaviruses, suggests that Au ra virus 26S RNA contains a packaging signal for incorporation into vi rions. The importance of the packaging of this RNA into virions in the natural history of the virus remains to be determined.