Contemporary issues in Tospovirus molecular biology
German TL
Department of Entomology, University of Wisconsin, Madison, WI 53706, USA.
Correspondence: tlg@entomology.wisc.edu
In 1984, Francki noted the similarity between Tomato spotted wilt virus (TSWV) and viruses in the family Bunyaviridae: a large group of membrane-bound, mostly arthropod-transmitted, animal-infecting viruses with tripartite negative-stranded RNA genomes. Data obtained in the last two decades elucidating the molecular biology of TSWV and the closely related Impatiens necrotic spot virus confirmed this observation and supplied the rational for the creation of the Tospovirus genus to include the plant-infecting, thrips-transmitted members of the family Bunyaviridae. Virions of TSWV, the type member of the family, are 80¨C120 nm pleomorphic particles with surface projections composed of two viral glycoproteins, GN and GC. The genome consists of three negative or ambisense ssRNAs designated S (2.9 kb), M (4.8 kb) and L (8.9 kb) with partially complementary terminal sequences that allow them to form a pseudocircular or panhandle conformation and likely serve as promoters for RNA synthesis. Each genomic RNA is encapsidated by multiple copies of the viral nucleocapsid protein (N) to form ribonucleocapsid structures (RNPs). The RNPs along with an estimated 20 copies of the L protein (RNA-dependent RNA polymerase) are enclosed in the viral membrane that is acquired during maturation at the Golgi. The ambisense S RNA encodes a 52.4 kDa nonstructural protein (NSs), in the viral (v) sense and the 29 kDa N protein in the viral complementary (vc) sense. The NSs protein is expressed from a v sense 1.7 kb subgenomic mRNA and the N protein from a 1.2 kb vc sense subgenomic RNA. The M RNA encodes a 33.6 kDa movement protein in the v sense and a 127.4 kDa precursor to GN and GC in the vc sense and relies on an expression strategy similar to that of the S RNA. The completely negative sense RNA contains a single open reading frame in the vc sense that is expressed from a nearly genome length mRNA. An additional 10¨C20 nucleotides of host origin are found at the 5¡¯ ends of the viral N and NSs mRNAs providing evidence that TSWV uses the ends of host RNAs to acquire cap structures and direct transcription by means of a cap-snatching, typical of all segmented negative-sense RNA viruses. Details concerning the role of each viral protein in the infection cycle are beginning to emerge and recent work has suggested that, as expected, host proteins play a direct role as well. Future research directed towards understanding how proteins and nucleic acids interact to catalyze, regulate and target their activities will provide important new insights into the biology of these viruses.
