Functional and genetic analysis of coronavirus replicase-transcriptase proteins

• Published in: PLoS pathogens Vol. 1; no. 4; p. e39
• Main Authors: Sawicki, Stanley G, Sawicki, Dorothea L, Younker, Diane, Meyer, Yvonne, Thiel, Volker, Stokes, Helen, Siddell, Stuart G
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 01.12.2005; Public Library of Science (PLoS)
• Subjects: Animals; Coronaviridae - enzymology; Coronaviridae - genetics; DNA-Directed RNA Polymerases - genetics; Enzymes; Genetics/Gene Expression; Genetics/Gene Function; Infectious Diseases; Microbiology; Molecular Biology - Structural Biology; Mus (Mouse); Mutation; Proteins; Ribonucleic acid; RNA; RNA-Directed DNA Polymerase - genetics; Viral Proteins - genetics; Virology; Viruses
• ISSN: 1553-7366; 1553-7374; 1553-7374
• DOI: 10.1371/journal.ppat.0010039

The coronavirus replicase-transcriptase complex is an assembly of viral and cellular proteins that mediate the synthesis of genome and subgenome-sized mRNAs in the virus-infected cell. Here, we report a genetic and functional analysis of 19 temperature-sensitive (ts) mutants of Murine hepatitis virus MHV-A59 that are unable to synthesize viral RNA when the infection is initiated and maintained at the non-permissive temperature. Both classical and biochemical complementation analysis leads us to predict that the majority of MHV-A59 ORF1a replicase gene products (non-structural proteins nsp1-nsp11) form a single complementation group (cistron1) while the replicase gene products encoded in ORF1b (non-structural proteins nsp12-nsp16) are able to function in trans and comprise at least three, and possibly five, further complementation groups (cistrons II-VI). Also, we have identified mutations in the non-structural proteins nsp 4, nsp5, nsp10, nsp12, nsp14, and nsp16 that are responsible for the ts phenotype of eight MHV-A59 mutants, which allows us to conclude that these proteins are essential for the assembly of a functional replicase-transcriptase complex. Finally, our analysis of viral RNA synthesis in ts mutant virus-infected cells allows us to discriminate three phenotypes with regard to the inability of specific mutants to synthesize viral RNA at the non-permissive temperature. Mutant LA ts6 appeared to be defective in continuing negative-strand synthesis, mutant Alb ts16 appeared to form negative strands but these were not utilized for positive-strand RNA synthesis, and mutant Alb ts22 was defective in the elongation of both positive- and negative-strand RNA. On the basis of these results, we propose a model that describes a pathway for viral RNA synthesis in MHV-A59-infected cells. Further biochemical analysis of these mutants should allow us to identify intermediates in this pathway and elucidate the precise function(s) of the viral replicase proteins involved.