A general method for the induction and screening of antisera for cDNA-encoded polypeptides: antibodies specific for a coronavirus putative polymerase-encoding gene

A prokaryotic vector, pGE374, containing the recA and lacZ genes, out-of-frame, was used for the expression of cDNA derived from the putative polymerase-encoding gene of the coronavirus mouse hepatitis virus strain A59 (MHV-A59). The pGE374/viral recombinant vector generates a tripartite bacterial/viral protein composed of a segment of the RecA protein at the N terminus, the coronaviral sequences in the middle, and an enzymatically active beta-galactosidase at the C terminus. Rabbits immunized with such recombinant proteins generated antibodies to the MHV-A59 portion of the tripartite protein. Because the MHV-A59 polymerase proteins have been difficult to identify during infection, we used a novel method to demonstrate the viral specificity of the antiserum. The viral cDNA was excised from the expression vector, and transferred to a pGem vector, downstream from and in-frame with a portion of the cat gene. This construct contained a bacteriophage RNA polymerase promoter that enabled the cell-free synthesis of a fusion protein that was used to verify that antibodies were generated to the expressed viral DNA. This strategy was shown to successfully result in the specific generation of antibodies to the encoded information of the viral cDNA. Furthermore, this method has general applicability in the generation and characterization of antibodies directed against proteins encoded in cDNAs.

Synthesis of virus-specific RNA in permeabilized murine coronavirus-infected cells

We have developed a permeabilized cell system for assaying mouse hepatitis virus-specific RNA polymerase activity. This activity was characterized as to its requirements for mono- and divalent cations, requirements for an exogenous energy source, and pH optimum. This system faithfully reflects MHV-specific RNA synthesis in the intact cell, with regard to both its time of appearance during the course of infection and the products synthesized. The system is efficient and the RNA products were identical to those observed in intact MHV-infected cells as judged by agarose gel electrophoresis and hybridization. Permeabilized cells appear to be an ideal system for studying coronavirus RNA synthesis since they closely mimic in vivo conditions while allowing much of the experimental flexibility of truly cell-free systems.

Detection of a murine coronavirus nonstructural protein encoded in a downstream open reading frame

Mouse hepatitis virus (MHV) gene 5 contains two open reading frames. We have expressed the second open reading frame of this gene (gene 5 ORF 2) in an Escherichia coli expression system. This system utilized a plasmid which contained the promoter and the first 36 codons of the recA gene fused in frame with the MHV gene 5 ORF 2, which is fused in turn to the beta-galactosidase gene. The protein product of this gene fusion was used to raise antibody to gene 5 ORF 2. The specificity of the antibody was verified by immunoprecipitation of the in vitro transcribed and translated protein product of gene 5 ORF 2. The second reading frame of MHV gene 5 was shown to be expressed during the course of infection by immunocytochemistry and radioimmunoprecipitation using the antibody raised against the E. coli fusion protein and by two-dimensional gel electrophoresis.

Genetic analysis of murine hepatitis virus strain JHM

We performed a genetic analysis of 37 temperature-sensitive mutants of murine hepatitis virus strain JHM. Of our mutants, 32 did not induce murine hepatitis virus-specific RNA synthesis in infected cells at the restrictive temperature, 39 degrees C. By complementation testing we have identified at least seven nonoverlapping complementation groups. Six of the genes identified in this way are required for murine hepatitis virus-specific RNA synthesis. The seventh complementation group is made up of five mutants which induced virus-specific RNA synthesis at 39 degrees C.