FIG 1 .

MHV genome organization and nsp14 exoribonuclease motifs. (Top) The MHV genome is a 31.4-kb, capped (dark circle), and polyadenylated positive-sense RNA molecule. The first two-thirds of the genome encode 16 nonstructural proteins translated as a single polyprotein with a ribosomal frameshift. The final one-third encodes the structural and accessory proteins. (Inset) Nsp14 encodes an exoribonuclease (solid blue) and an N7-methyltransferase (hatched blue) and has 3 zinc fingers (gray boxes) predicted from the solved SARS nsp10/14 crystal structure (PDB 5C8U) (44). Catalytic residues for ExoN are marked with white boxes, and the engineered mutations for MHV-ExoN(-) are shown below the genome. The nsp12 RNA-dependent RNA polymerase is highlighted in red.