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There are no known host cell enzymes that can copy the genomes of RNA viruses. However, at least one enzyme, RNA polymerase II, can copy an RNA template. The 1.7-kb circular, ssRNA genome of hepatitis delta satellite virus is copied by RNA polymerase II to form multimeric RNAs (see the figure). How RNA polymerase II, an enzyme that produces pre-mRNAs from DNA templates, is reprogrammed to copy a circular RNA template is not known.

Hepatitis delta satellite (–) strand genome RNA is copied by RNA polymerase II at the indicated position. The polymerase passes the poly(A) signal (purple box) and the self-cleavage domain (red circle). For more information, see Fig. 6.25 Redrawn from Taylor JM. 1999. Curr Top Microbiol Immunol 239:107–122, with permission.

Figure 3.6 Structure and expression of viral single-stranded (+) RNA genomes. (A) Synthesis of genomes, mRNA, and protein. (B) Genome configurations. UTR, untranslated region; VP_g, virion protein, genome linked.