Characterization of the Nucleic Acid Binding Properties of Tomato Spotted Wilt Virus Nucleocapsid Protein

Tomato spotted wilt tospovirus (TSWV) is the type member of the plant-infecting viruses of the genusTospovirusin the family Bunyaviridae. The three TSWV RNAs are encapsidated with nucleocapsid (N) protein to form ribonucleoprotein (RNP) which serves as the template for viral transcription and replication. Regions of the open reading frame coding for the N protein on the small (S) RNA were subcloned into pET protein expression vectors and expressed inEscherichia coliBL21 (DE3) cells. Full-length N, N amino and carboxy halves, and two N carboxy-terminal regions were expressed and isolated by metal chelate affinity chromatography. The N protein, both of its halves and the extreme carboxy-terminal region, bound cooperatively and irrespective of sequence to radiolabeled single-stranded RNA produced by runoff transcription of clones of either TSWV S RNA or cowpea chlorotic mottle virus RNA3. N protein did not bind to radiolabeled double-stranded TSWV RNA. The density of the synthetic RNase-sensitive N protein–RNA complexes was 1.32 g/ml, similar to the density of authentic Bunyaviridae RNPs. These studies are the first to indicate differences in the nucleic acid binding abilities ofTospovirusandHantavirusnucleocapsid proteins, the only characterized nucleocapsid proteins of the family Bunyaviridae.