Describe the general genome organization of poliovirus and the fate of its polyprotein.

• A. A ~7.5 kb negative-sense RNA genome with multiple ORFs encoding separate proteins.
• B. A segmented genome with 8 genes separated by introns.
• C. A ~7.5 kb positive-sense RNA genome with a single ORF encoding a polyprotein that is cleaved into structural proteins VP1-4 and non-structural replication proteins.
• D. A DNA genome of about 8 kb with no polyprotein.

Answer: (C)

Poliovirus has a positive-sense, single-stranded RNA genome about 7.5 kb in length that can act directly as mRNA in the host cell. It carries one long open reading frame that is translated into a single polyprotein. This polyprotein is then cleaved by viral proteases into functional proteins: the structural capsid proteins VP1–VP4 and the non-structural replication proteins. There is no genome segmentation and no introns. Translation is driven by an internal ribosome entry site in the 5' untranslated region, and the genome begins with a VPg protein that serves as a primer for RNA synthesis. The mature proteins then assemble into virions or form the replication complex needed for RNA replication in the cytoplasm.