AURA ALPHAVIRUS SUBGENOMIC RNA IS PACKAGED INTO VIRIONS OF 2 SIZES

The alphavirus genome is 11.8 kb in size. During infection, a 4.2-kb, subgenomic RNA is also produced, Most alphaviruses package only the ge nomic RNA into virions, which are enveloped particles with icosahedral symmetry, having a triangulation number (T)=4. Aura virus, however, p ackages both the genomic RNA and the subgenomic RNA into virions. The genomic RNA is primarily packaged into a virion that has a diameter of 72 nm and which appears to be identical to the virions produced by ot her alphaviruses. The subgenomic RNA is packaged into two major, regul ar particles with diameters of 72 and 62 nm. The 72-nm-diameter partic le appears to be identical in construction to virions containing genom ic RNA. The 62-nm-diameter particle probably has T=3. The large and sm all Aura virions can be partially separated in sucrose gradients. In a ddition to these two major classes of particles, there are other parti cles produced that appear to arise from abortive assembly. From these results and from previous studies of alphavirus assembly, we suggest t hat during assembly of alphavirus nucleocapsids in the infected cell t here is a specific initiation event followed by recruitment of additio nal capsid subunits into the complex, that the triangulation number of the complex is not predetermined but depends upon the size of the RNA and interactions that occur during assembly, and that budding of asse mbled nucleocapsids results in the acquisition of an envelope containi ng glycoproteins arranged in a manner determined by the nucleocapsid.