NUCLEOTIDE-SEQUENCE OF THE N-GENE OF WATERMELON SILVER MOTTLE VIRUS, A PROPOSED NEW MEMBER OF THE GENUS TOSPOVIRUS

A virus isolated from watermelon was previously identified as tomato spotted wilt-like virus in the genus Tospovirus of the family Bunyaviridae. The virus, designated Tospo-W, contains a nucleocapsid (N) protein serologically unrelated to tomato spotted wilt virus (TSWV) or impatiens necrotic spot virus (INSV). In this study, double-stranded RNAs of Tospo-W were extracted from virus-infected tobacco and used for cDNA cloning. A cDNA clone, pWN12, that was selected from a cDNA library to the separated S dsRNA, contained 1,200 nucleotides including the entire N ORF that encodes a protein of 275 amino acids with a predicted M(r) of 30,657. Comparison of the nucleic acid sequence of the Tospo-W N gene with those of the other five tospoviruses revealed that their identity percentages were 54.4-55.9%. The degrees of similarity at the amino acid level were only 39.6-46.1%. In vitro transcription and translation of the clone pWN12 generated a protein of 31 kDa, similar to the authentic N protein and reacted specifically with the antibody to the Tospo-W N protein. A nick-translated DNA probe from the insert of pWN12 reacted with RNAs extracted from plants infected with Tospo-W or with a tospovirus isolate from melon, but not with RNAs from plants infected with three TSWV isolates or three INSV isolates. The low-degree homology of the N gene with other tospoviruses coupled with the lack of cross hybridization using a Tospo-W-specific cDNA probe suggests that Tospo-W, renamed as watermelon silver mottle virus, is a distinct member of the genus Tospovirus.