Fig. 14.23

Genome organization and replication in coronaviruses (here SARS-related coronavirus). The viral RNA genome functions as a mRNA, and is translated in the cytoplasm. The two overlapping reading frames encode non-structural polyproteins 1a and 1ab (pp1a and pp1ab). A hairpin structure induces a frameshifting during translation that leads in about 15–20 % of translation processes to the synthesis of non-structural polyprotein 1ab, which harbours in its carboxy-terminal region the RNA-dependent RNA polymerase activity. It is cleaved by the autocatalytic activity of a cysteine protease in the centre of the precursor protein. Papain – like protease sequences reside within the N-terminal domains of proteins 1a and 1ab, they are autocatalytic active and perform their own cleavage from the precursor proteins. The RNA-dependent RNA polymerase transcribes the positive-sense RNA genome into a complementary negative strand. It serves as a template for the synthesis of new positive-sense RNA genomes and for the transcription of various subgenomic mRNA species, which are modified at the 5′ end with a cap structure, and include identical sequences of the leader region in all mRNA molecules. The structural proteins are translated from the different subgenomic mRNAs, whose reading frames are localized in the 3′-oriented part of the positive-sense RNA genome. They are partially overlapping. Besides the genes specified here, there are some additional small reading frames in this region of the genome in the different coronavirus types which commonly encode non-structural proteins of unknown function. They are not shown here