Summary auto-generated
Milk vetch dwarf virus (MDV) is a nanovirus with a multipartite genome consisting of ten circular single-stranded DNA components of approximately 1 kb each. Researchers cloned and sequenced all ten components (C1–C10) from the N isolate of MDV, which infects legume crops in Japan via the aphid vector Aphis craccivora. Each DNA component contains one open reading frame encoding a protein larger than 10 kDa, plus a conserved stem–loop structure in the non-coding region. Four components (C1, C2, C3, C10) encode replication-associated (Rep) proteins sharing only 42–57% amino acid identity, while six others encode non-Rep proteins of 12–19 kDa. Sequence comparisons reveal close relationships to faba bean necrotic yellows virus (FBNYV) and subterranean clover stunt virus (SCSV), suggesting MDV, FBNYV, and SCSV diverged from a common ancestor with multiple Rep components. The putative protein from C9 is identified as the capsid protein based on sequence homology with FBNYV. Notably, the C4-encoded protein contains a retinoblastoma-binding motif, suggesting involvement in host cell cycle control—a function typically associated with geminivirus Rep proteins but here distributed across separate nanovirus components.
Key findings
- MDV genome comprises ten distinct circular ssDNA components, each encoding one major ORF, with four components encoding structurally distinct Rep proteins sharing only 42–57% amino acid identity
- MDV is most closely related to FBNYV (nucleotide identity 78–79%, amino acid identity 83–91% for six putative proteins), supporting classification as a separate viral species
- Only the virus-sense strand is encapsidated in particles; the virus does not multiply within aphid vectors
- The MDV-C4 encoded protein contains a conserved retinoblastoma-binding motif (LXCXE), suggesting independent regulation of host cell cycle rather than through Rep proteins as in geminiviruses
- Multiple Rep-encoding components are stably maintained in the isolate over 15 years, though evidence suggests not all may be essential for replication
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Abstract
Milk vetch dwarf virus (MDV) is a member of the proposed genus Nanovirus, and its genome is composed of multiple, circular ssDNA components of about 1 kb. We have cloned and sequenced ten ssDNA components and designated them MDV-C1 to C10. Each DNA component contains one potential major open reading frame, and contains a putative stem-loop structure in the non-coding region. Notably, four components (C1, C2, C3 and C10) encode distinct replication-associated (Rep) proteins of 33 kDa, which show only limited (42-57%) amino acid identity. The six other components encode proteins with calculated molecular masses ranging from 12.7 to 19.7 kDa. Comparison of the sequences with those of other nanoviruses reveals that MDV is closely related to faba bean necrotic yellows virus (FBNYV) and subterranean clover stunt virus (SCSV). Six putative MDV genome products, including one Rep and five non-Rep proteins, show high (70.4-90.9%) amino acid identity to the corresponding six FBNYV proteins, whereas two other Rep proteins encoded by MDV-C2 and C3 are 82.3% and 73.0% identical to those encoded by SCSV-C2 and C6, respectively. These results indicate that MDV, FBNYV and SCSV have diverged from a common origin, which had multiple Rep components. In addition, the putative proteins encoded by MDV-C4 and its homologues contain a consensus retinoblastoma-binding motif, suggesting that they may be involved in controlling the host cell cycle.