tospoviruses, however, an ambisense coding strategy is used for the two proteins (ambi = both). In this strategy, the two genes encoded in a genomic segment are linked tail to tail so that they are in different polarities, as illustrated in Fig. 4.20. The gene for N is present at the 3' end of the genome S segment in the minus-sense orientation, and synthesis of the mRNA for N occurs from the genome segment. Expression of this gene occurs early because its mRNA is synthesized from the entering genome by the polymerase activity present in viral nucleocapsids. The gene for NSs is plus sense within the genome, but the genomic RNA does not serve as mRNA. Instead, an mRNA for NSs is synthesized from the antigenomic RNA. Thus, NSs is expressed late because its mRNA can only be made after replication of the incoming genomic RNA to produce the antigenomic RNA. Termination of either mRNA occurs at a secondary structure between the genes for N and NS, which appears to cause the polymerase to fall off and release the mRNA.
Two glycoproteins, usually called G1 and G2, are translated from mRNA made from M (Fig. 4.19). They are thought to be produced as a polyprotein that is cleaved by a cellular protease to separate the two glycoproteins, analogous to what happens in some of the (+)RNA viruses that have envelopes (e.g., coronaviruses and flaviviruses). However, there is also evidence that reinitiation to produce the second glycoprotein can occur, at least in some viruses.
The M segments of hantaviruses and nairoviruses encode only the two glycoproteins, but in the other three genera M encodes a third protein called NSm (Fig. 4.19). In phle-
Q mRNA A synthesis
^^ Replication mRNA ^
vcRNA Genome RNA
w synthesis w1
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