Fig. 14.21

Genome organization and replication in arteriviruses. The viral RNA genome functions as mRNA, and is translated in the cytoplasm. The two overlapping reading frames encode non-structural polyproteins 1a and 1ab. A hairpin structure induces a ribosomal frameshift during translation that leads in approximately 15–20 % of all translation processes to the synthesis of non-structural polyprotein 1ab, which contains the RNA-dependent RNA polymerase domain in its carboxy-terminal region. Both polyproteins are cleaved by the autocatalytic activity of two cysteine proteases and a serine protease that are located in the amino-terminal regions of the precursor proteins. The RNA-dependent RNA polymerase transcribes the positive-sense RNA genome into a negative-sense RNA strand. This serves both as a template for the synthesis of new positive-sense RNA genomes and for transcription of a series of subgenomic mRNA species, which are modified at the 5′ end with a cap group and contain identical leader sequences in all mRNA molecules. The structural proteins are translated from the different subgenomic mRNAs, whose reading frames are localized in the 3′-terminal third of the positive-sense RNA genome. They partially overlap