Positive Stranded RNA Viruses Make Polyproteins

Picornaviruses are small, spherical single-stranded RNA viruses. They include polio, common cold, hepatitis A, and foot and mouth disease. Their genome is long enough for about a dozen genes. Since they are positive-strand RNA viruses, their RNA can be directly used as mRNA. The viral RNA does have a poly(A) tail attached to its 3'-end but has no cap. Instead a protein, the Vpg protein, is attached covalently to the 5'-

FIGURE 17.17 Rotaviruses Have Multiple Double-Stranded RNA Molecules

Rotavirus particles have two protein coats that encapsulate about a dozen double-stranded RNA molecules. Each of the dsRNA pieces encodes one of the proteins necessary for viral survival and propagation.

FIGURE 17.17 Rotaviruses Have Multiple Double-Stranded RNA Molecules

Rotavirus particles have two protein coats that encapsulate about a dozen double-stranded RNA molecules. Each of the dsRNA pieces encodes one of the proteins necessary for viral survival and propagation.

Polyproteins are made from a single giant gene and then cut up to give several final proteins.

end of the virus RNA. However, there is a technical problem. Unlike bacteria where a single mRNA molecule may code for several proteins (an operon; see Ch. 6), in higher organisms each molecule of mRNA only encodes a single protein. Eukaryotic ribosomes will only translate the first reading frame on an RNA message, even if it carries several. Picornaviruses do indeed use their positive ssRNA molecule directly as a messenger RNA. They avoid the problem by using the RNA to code for a single giant polypeptide that uses all of their genetic information (Fig. 17.18). This "polyprotein" is then chopped up into 10 to 20 smaller proteins.

The RNA in negative strand RNA viruses is the antisense stand.

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