Fig. 14.8
From: Viruses with Single-Stranded, Positive-Sense RNA Genomes
Genome organization of caliciviruses. (a) Norovirus (Norwalk virus). (b) Lagovirus (rabbit haemorrhagic disease virus). (c) Vesivirus (feline calicivirus). (d) Sapovirus (Sapporo virus). The mRNA genome of caliciviruses is modified by a covalently linked protein (Vpg) at the 5′ end, and is polyadenylated at the 3′ terminus. It is translated in the cytoplasm. The organization of the open reading frames differs among the different genera: in noroviruses and vesiviruses there are three open reading frames. ORF1 is separated from ORF2 and is responsible for the synthesis of the precursor for the non-structural proteins. It is autocatalytically cleaved by the serine protease 3CLpro into Vpg, an NTPase (helicase), RNA-dependent RNA polymerase and two additional non-structural proteins (p48 and p22 in noroviruses). The non-structural protein precursors of lagoviruses, sapoviruses and vesiviruses contain the respective protein functions, but they have been scarcely examined. The RNA polymerase is responsible for the synthesis of the negative-sense RNA molecules, from which the RNA genomes of progeny viruses are synthesized during the replication cycle. ORF1 encodes capsid protein VP1 and ORF3 contains the genetic information for VP2, a protein associated with the RNA genome. In lagoviruses and sapoviruses, the reading frame encoding the non-structural protein precursor is fused in frame to the open reading frame of capsid protein VP1. The small ORF2 encodes VP2