The core region of the coat protein gene is highly useful for establishing the provisional identification and classification of begomoviruses

Polymerase chain reaction (PCR) was applied to detect and establish provisional identity of begomoviruses through amplification of a approximately 575 bp fragment of the begomoviral coat protein gene (CP), referred to as the 'core' region of the CP gene (core CP). The core CP fragment contains conserved and unique regions, and was hypothesized to constitute a sequence useful for begomovirus classification. Virus relationships were predicted by distance and parsimony analyses using the A component (bipartite viruses) or full genome (monopartite viruses), CP gene, core CP, or the 200 5'-nucleotides (nt) of the CP. Reconstructed trees and sequence divergence estimates yielded very similar conclusions for all sequence sets, while the CP 5'-200 nt was the best strain discriminator. Alignment of the core CP region for 52 field isolates with reference begomovirus sequences permitted provisional virus identification based on tree position and extent of sequence divergence. Geographic origin of field isolates was predictable based on phylogenetic separation of field isolates examined here. A 'closest match' or genus-level identification could be obtained for previously undescribed begomoviruses using the BLAST program to search a reference core CP database located at our website and/or in GenBank. Here, we describe an informative molecular marker that permits provisional begomovirus identification and classification using a begomoviral sequence that is smaller than the presently accepted, but less accessible CP sequence.

Three epitopes located on the coat protein amino terminus of viruses in the bean common mosaic potyvirus subgroup

Twenty-seven of 29 strains of viruses in the bean common mosaic virus (BCMV) subgroup of legume-infecting potyviruses reacted strongly with one or more of the monoclonal antibodies (MAbs) which are known to be specific for epitopes located along the 50 amino acids which constitute the N-terminal end of the viral coat protein. Approximately one half of the virus strains reacted with the N-terminal epitope specific (NTES) MAb 4G12 which is specific for epitope E/B4, while the other half reacted with NTES MAbs 4A1 or 4F9 which are specific for epitope E/B3. All but two strains contained at least one of these epitopes while no strain contained both. Competitive assays using five sequential, non-overlapping, synthetic, 10mer peptides indicated that the amino acids critical for epitope E/B3 reaction were located at positions 5, 7, and 10 from the N-terminal end of the coat protein. By deduction we postulate that the amino acids critical for epitope E/B4 are located at positions 10, 16, and 17. Because epitope E/B3 requires isoleucine at position 10 for expression whereas epitope E/B4 requires valine to be expressed, no one strain can express both epitopes. Two viruses in our tests (azuki mosaic and Dendrobium mosaic viruses) had deletions in this portion of their sequence explaining their failure to react MAbs specific for either epitope. The critical amino acids for a third epitope, E/B3A, were located at positions 16 and 17. We found no correlation between any of the three N-terminal epitopes defined in this study and the presence or absence of any biological property that we could accurately measure: i.e., symptomatology, host range, or pathotype. However, when coat protein sequences were aligned according to epitope type E/B3 or E/B4, we found that sequences within groups had high levels of identity while between group identities were low. We also found that sequences in the 3'-end non-coding region exhibited similar relationships within and between epitope groups. Two strains of BCMV (NL-4 and RU-1) were found to possess coat protein sequences typical of epitope E/B4 but 3'-NCR sequences typical of epitope E/B3. These data suggest that both strains may be the result of natural recombinants between the two epitope groups.

Phylogenetic analysis of the Potyviridae with emphasis on legume-infecting potyviruses

The 3'-terminal nucleotide sequences of thirteen authenticated strains of bean common mosaic virus (BCMV) and one strain of bean common mosaic necrosis virus (BCMNV) were obtained. The regions sequenced included the coat protein coding sequence and 3'-end non-coding region. These data, combined with sequence information from other legume-infecting potyviruses and the Potyviridae were used for phylogenetic analysis. Evidence is provided for delineation of BCMNV as distinct from BCMV and the inclusion of azuki mosaic, dendrobium mosaic, blackeye cowpea mosaic, and peanut stripe viruses as strains of BCMV. This relationship defines the members of the BCMV and BCMNV subgroups. These data also provide a basis upon which to define virus strains, in combination with biological data. Other aspects and implications of legume-infecting potyvirus phylogenetics are discussed.

In vitro destabilization of plant viruses and cDNA synthesis

DNA copies of a wide range of RNA viruses can be made by the direct addition of appropriately treated, purified virus particles to a reverse transcription reaction. Therefore, many problems associated with RNA isolation can be circumvented. Virus particles can be sufficiently destabilized by adjustments of salt content, buffer, pH or by the use of physical force supplied by a freeze/thaw cycle so that RNA in sufficient quantity and physical condition is available for the synthesis of in some cases, full length cDNAs. cDNAs have been made of viruses in the bromo-, poty-, carla-, ilar-, potex-, tobra and tobamovirus groups. Reported here are experiments with cowpea chlorotic mottle virus and bean common mosaic virus.