Melon yellow spot virus: A Distinct Species of the Genus Tospovirus Isolated from Melon.

ABSTRACT A tospovirus-like virus recovered from netted melon was transmitted by Thrips palmi in a persistent manner but had different cytopathological features from tospoviruses previously reported. Viral nucleocapsid (N) was purified with two protective reagents, 2-mercaptoethanol and L-ascorbic acid, and RNA extracted from the viral nucleocapsid was used for genomic analysis. The virus had a genome consisting of three single-stranded RNA molecules. The open reading frame on the viral complementary strand, located at the 3' end of the viral S RNA, encoded the N protein. The 3' terminus of this RNA also contained an eight-nucleotide sequence similar to the conserved sequence at the 3' end of genomic RNA molecules of tospoviruses. These features of the viral genome are identical to those of tospoviruses; therefore, this virus is considered to belong to the genus Tospovirus. Its N protein comprised 279 amino acids and had a molecular mass of 31.0 kDa. Comparisons of its amino acid sequence with those of known tospoviruses revealed less than 60% identity. This melon virus is concluded to be a distinct species in the genus Tospovirus, and the name Melon yellow spot virus is proposed.

Detection of Cymbidium Mosaic Potexvirus and Odontoglossum Ringspot Tobamovirus from Thai Orchids by Rapid Immunofilter Paper Assay.

During 1992 to 1994, 442 orchid plants from 24 genera in 43 nurseries and locations in eight provinces of northern, central, and southern regions of Thailand were examined for Cymbidium mosaic potexvirus (CyMV) and Odontoglossum ringspot tobamovirus (ORSV) with rapid immunofilter paper assay (RIPA). Both viruses were confirmed in all three regions. However, CyMV was detected in 17 genera in 93% nurseries, while ORSV was detected in only four genera (Arachnis, Cattleya, Oncidium, and Vanda) in 40% of nurseries tested. These results indicate that CyMV is more prevalent than ORSV in Thailand. Mixed infections of ORSV and CyMV were often found, and double infections were observed in 95% of ORSV-infected plants. The virus infections were divided into two categories: infection by CyMV alone and double infection by both viruses in Cattleya and Oncidium. CyMV was often detected alone from symptomatic or asymptomatic plants of certain orchid genera. In Oncidium, however, CyMV was detected alone from 79% of plants without symptoms, while both viruses were detected from 90% of plants with mosaic and necrotic spot symptoms on leaves. Expression of symptoms may be promoted by the double infection of CyMV and ORSV in Oncidium.

Comparison of the S RNA segments among Japanese isolates and Taiwanese isolates of Watermelon silver mottle virus.

The complete nucleotide sequences of the S RNA of two Japanese isolates of Watermelon silver mottle virus were determined. One was isolated from naturally infected watermelon and causes malformation on upper leaves of Tetragonia expansa. The other was isolated from melon and causes characteristic yellow necrotic lesions on upper leaves of T. expansa. The total nucleotide sequences of the S RNA of WS-O and WS-Y were 3553 nt and 3558 nt long, respectively. Both the nucleotide sequences and the deduced amino acid sequences of WS-O and WS-Y were quite similar even though the symptoms on T. expansa are quite different. They were also significantly similar to those of the Taiwanese isolates, Topso-W and Tospo-To. These results suggested that the Japanese isolates and the Taiwanese isolates of WSMoV were classified as one group not only serologically but also genetically. Within the S RNA sequences, the most variable region was the intergenic region between the N gene and the NSs gene. This was due to a 20 nt insertion between the Japanese isolates and the Taiwanese isolates.

Sequence analysis of RNA-2 of different isolates of broad bean wilt virus confirms the existence of two distinct species.

The complete nucleotide sequence of RNA-2 from a Japanese isolate IP of broad bean wilt virus (BBWV) was determined. The sequence encodes a single large polyprotein, which contains a putative movement protein and two coat proteins (CPs). The 3'-terminal sequences of RNA-2 were also determined for three other Japanese isolates and two ATCC isolates (PV132 and PV176) of BBWV. The CPs of the four Japanese isolates share 86.8-98.0% amino acid sequences homology with one another and 88.3-96.5% with those reported for the isolate PV131 (BBWV-2). However, they have only 57.9-66.2% homology with those of PV132 and PV176 (BBWV-1).

The complete sequence of soybean chlorotic mottle virus DNA and the identification of a novel promoter.

The complete nucleotide sequence of an infectious clone of soybean chlorotic mottle virus (SoyCMV) DNA was determined and compared with those of three other caulimoviruses, cauliflower mosaic virus (CaMV), carnation etched ring virus and figwort mosaic virus. The double-stranded DNA genome of SoyCMV (8,175 bp) contained nine open reading frames (ORFs) and one large intergenic region. The primer binding sites, gene organization and size of ORFs were similar to those of the other caulimoviruses, except for ORF I, which was split into ORF Ia and Ib. The amino acid sequences deduced from each ORF showed only short, highly homologous regions in several of the corresponding ORFs of the three other caulimoviruses. A promoter fragment of 378 bp in SoyCMV ORF III showed a strong expression activity, comparable to that of the CaMV 35S promoter, in tobacco mesophyll protoplasts as determined by a beta-glucuronidase assay using electrotransfection. The fragment contained CAAT and TATA boxes but no transcriptional enhancer signal as reported for the CaMV 35S promoter. Instead, it had sequences homologous to a part of the translational enhancer signal reported for the 5'-leader sequence of tobacco mosaic virus RNA.