INFECTIOUS BOVINE VIRAL DIARRHEA VIRUS (STRAIN NADL) RNA FROM STABLE CDNA CLONES - A CELLULAR INSERT DETERMINES NS3 PRODUCTION AND VIRAL CYTOPATHOGENICITY

Bovine viral diarrhea virus (BVDV), strain NADL, was originally isolat ed from an animal with fatal mucosal disease. This isolate is cytopath ic in cell culture and produces two forms of NS3-containing proteins: uncleaved NS2-3 and mature NS3. For BVDV NADL, the production of NS3, a characteristic of cytopathic BVDV strains, is believed to be a conse quence of an in-frame insertion of a 270-nucleotide cellular mRNA sequ ence (called cIns) in the NS2 coding region. In this study, we constru cted a stable full-length cDNA copy of BVDV NADL in a low-copy-number plasmid vector. As assayed by transfection of MDBK cells, uncapped RNA s transcribed from this template were highly infectious (>10(5) PFU/mu g). The recovered virus was similar in plaque morphology, growth prop erties, polyprotein processing, and cytopathogenicity to the BVDV NADL parent. Deletion of cIns abolished processing at the NS2/NS3 site and produced a virus that was no longer cytopathic for MDBK cells. This d eletion did not affect the efficiency of infectious virus production o r viral protein production, but it reduced the level of virus-specific RNA synthesis and accumulation. Thus, cIns not only modulates NS3 pro duction but also upregulates RNA replication relative to an isogenic n oncytopathic derivative lacking the insert. These results raise the po ssibility of a linkage between enhanced BVDV NADL RNA replication and virus-induced cytopathogenicity.