The Bunyamwera virus nonstructural protein NSs inhibits viral RNA synthesis in a minireplicon system

The small (S) genomic segment of Bunyamwera virus (family Bunyaviridae, gen us Bunyavirus) encodes the nucleocapsid protein, N, and a nonstructural pro tein, NSs, in overlapping reading frames. In order to elucidate the functio n of NSs, we established a plasmid-based minireplicon system using mammalia n cells that express large amounts of T7 RNA polymerase. Expression of N, t he viral polymerase protein (L), and a minireplicon containing a reporter g ene was sufficient to reconstitute functional virus nucleocapsids. Coexpres sion of NSs, however, led to a dose-dependent decrease in reporter activity without affecting expression of controls. The inhibition could not be reve rsed by overexpression of N, L or the minireplicon, indicating that the NSs effect was not caused by a reduction in virus gene expression. The NSs pro teins of two other members of the Bunyavirus genus, Guaroa virus and Lumbo virus, were also inhibitory in our system. The intracellular localisation o f Bunyamwera virus NSs was investigated and found to be predominantly cytop lasmic, but intranuclear inclusion was also detected. Taken together, these data suggest that, in mammalian cells, the bunyavirus NSs protein controls the activity of the viral polymerase by a highly conserved mechanism. (C) 2001 Academic Press.