Bunyamwera bunyavirus nonstructural protein NSs is a nonessential gene product that contributes to viral pathogenesis

Bunyamwera virus (family Bunyaviridae, genus Bunyavirus) contains a tripart ite negative-sense RNA genome. The smallest RNA segment S, encodes the nucl eocapsid protein N and a nonstructural protein, NSs, in overlapping reading frames. We have generated a mutant virus tacking NSs, called BUNdelNSs, by reverse genetics. Compared with the wild-type (wt) virus, BUNdelNSs exhibi ted a smaller plaque size and generated titers of virus approximately 1 log lower. In mammalian cells, the mutant expressed greatly increased levels o f N protein; significantly, the marked inhibition of host cell protein synt hesis shown by wt virus was considerably impaired by BUNdelNSs. When inocul ated by the intracerebral route BUNdelNSs killed BALB/c mice with a slower time course than wt and exhibited a reduced cell-to-cell spread, and titers of virus in the brain were tower. In addition, the abrogation of NSs expre ssion changed Bunyamwera virus from a noninducer to an inducer of an interf eron-beta promoter. These results suggest that, although not essential for growth in tissue culture or in mice, the bunyavirus NSs protein has several functions in the virus life cycle and contributes to viral pathogenesis.