Processing of open reading frame 1a replicase proteins nsp7 to nsp10 in murine hepatitis virus strain A59 replication

Coronaviruses express open reading frame 1a (ORF1a) and ORF1b polyproteins from which 16 nonstructural proteins (nsp) are derived. The highly conserved region at the carboxy terminus of ORF1a is processed by the nsp5 proteinase (Mpro) into mature products, including nsp7, nsp8, nsp9, and nsp10, proteins with predicted or identified activities involved in RNA synthesis. Although continuous translation and proteolytic processing of ORF1ab by Mpro is required for replication, it is unknown whether specific cleavage events within the polyprotein are dispensable. We determined the requirement for the nsp7 to nsp10 proteins and their processing during murine hepatitis virus (MHV) replication. Through use of an MHV reverse genetics system, in-frame deletions of the coding sequences for nsp7 to nsp10, or ablation of their flanking Mpro cleavage sites, were made and the effects upon replication were determined. Viable viruses were characterized by analysis of Mpro processing, RNA transcription, and growth fitness. Deletion of any of the regions encoding nsp7 to nsp10 was lethal. Disruption of the cleavage sites was lethal with the exception of that of the nsp9-nsp10 site, which resulted in a mutant virus with attenuated replication. Passage of the attenuated nsp9-nsp10 cleavage mutant increased fitness to near-wild-type kinetics without reversion to a virus capable of processing nsp9-nsp10. We also confirmed the presence of a second cleavage site between nsp7 and nsp8. In order to determine whether a distinct function could be attributed to preprocessed forms of the polyprotein, including nsp7 to nsp10, the genes encoding nsp7 and nsp8 were rearranged. The mutant virus was not viable, suggesting that the uncleaved protein may be essential for replication or proteolytic processing.