Fig. 25-2 Diagram of Alplmmrui, genome and its transcription and translation The plus strand virion RNA is capped and polyadenylated, and there are short nontranslated sequences at each terminus (single lines) The 5' two-thirds of the genome codes for nonstructural proteins and the 3' one-lhird for the structural proteins The portion coding for the nonstructural proteins is translated into a polyprolein which is cleaved info the four nonstructural proteins. Two of these fonn the RNA polymerase, which transcribes a full-length minus sense copy from the virion RNA, from which m turn two plus sense RNA species, virion RNA (not shown) and subgenomic mRNA, arc transcribed. The subgenomic mRNA, which is identical to the 3' one-third of the virion RNA, is translated into a polyprotein that is then processed into the viral structural pioteins El, E2, E3, 6K, and C
structural proteins This rnRNA is translated into anothei polyprotein which also has autoprotease activity, this viral enzyme together with certain cellular proteases cleaves the polyprotein to yield the four (or five) viral structural proteins: the nucleocapsid protein (C), and the peplomer glycoproteins (El, E2, and, in some species, E3), and a small transmembrane protein (6K). Genomic RNA and nucleoprotein are self-assembled into icosahedral nucleocap-sids in the cytoplasm, and these migrate to the plasma membrane. The peplomer proteins are glycosylated in stepwise fashion as they progress from the endoplasmic reticulum through the Golgi complex to the plasma membrane. Virion assembly takes place via budding of nucleocapsids through the modified plasma membrane in vertebrate cells, but through intracytoplasmic membranes in the case of invertebrate cells, in which alphaviruses are noncyt-olytic and establish persistent infection.
The replication of rubella virus is basically similar to that of the alpha-viruses with two significant exceptions, namely, that the rubella structural gene order is slightly different and rubella nucleocapsids acquire their envelope in mammalian cells by budding through intracytoplasmic membranes.
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