Cheravirus and Sadwavirus: two unassigned genera of plant positive-sense single-stranded RNA viruses formerly considered atypical members of the genus Nepovirus (family Comoviridae)

The genus Nepovirus (family Comoviridae) was known both for a good level of homogeneity and for the presence of atypical members. In particular, the atypical members of the genus differed by the number of capsid protein (CP) subunits. While typical nepoviruses have a single CP subunit with three structural domains, atypical nepoviruses have either three small CP subunits, probably corresponding to the three individual domains, or a large and a small subunit, probably containing two and one structural domains, respectively. These differences are corroborated by hierarchical clustering based on sequences derived from both genomic RNAs. Therefore, these atypical viruses are now classified in two distinct genera, Cheravirus (three CP subunits; type species Cherry rasp leaf virus) and Sadwavirus (two CP subunits; type species Satsuma dwarf virus).

A comprehensive open reading frame phylogenetic analysis of isometric positive strand ssRNA plant viruses

Rigorous large-scale whole genome comparisons are capable of providing more comprehensive and potentially more accurate descriptions of viral relationships, allowing for the effective validation and modification of current taxonomy. Using a set of 5 togaviruses as an outgroup, a comprehensive phylogeny for 115 isometric positive ssRNA plant viruses was generated based on the simultaneous comparison of over 480 ORFs found within completely sequenced genomes. With the exception of a diverse group of viruses representing the family Comoviridae, the single tree generated contained well supported branches corresponding to well established groups of viruses, including Bromoviridae, Umbravirus, Sobemovirus, and Tymoviridae. In addition, evidence for specific relationships between groups were also observed, specifically Tombusviridae + Umbravirus, and Luteoviridae + Sobemovirus. Various well established subgroups of viruses were also well resolved within the tree. In addition, some recent proposals involving the creation of new genera or the inclusion of newly described viruses into established genera were supported, while others were not. The evidence for frequent gene sharing and the potential consequences to viral taxonomy are discussed.

Nucleotide sequence analysis of RNA-2 of a flat apple isolate of Cherry rasp leaf virus with regions showing greater identity to animal picornaviruses than to related plant viruses

RNA-2 of a flat apple isolate of Cherry rasp leaf virus (CRLV-FA) appears to consist of 3274 nucleotides, excluding a 3' poly (A) tail. The data supports re-classification of CRLV in a new genus in the family Comoviridae. A single open reading frame (ORF) encoding a putative 108 kDa polyprotein was identified. Potential protease cleavage sites were identified which would result in the production of a putative movement protein (41 kDa), and 3 capsid protein subunits (24, 20, and 22 kDa, respectively). A 5'-UTR and 3'-UTR were identified, 248 nt and 146 nt long, respectively. The genome organisation of CRLV-FA RNA-2 is similar to that of Apple latent spherical virus (ALSV) RNA-2, a new member of the family Comoviridae. The Vp25 amino acid sequences were unique to CRLV-FA and ALSV (54% identity), with no relationship identified to any other virus. CRLV-FA Vp20 and Vp24 amino acid sequences were closely related to ALSV (59 and 65%, respectively) but the only other relationships identified were with a range of animal ssRNA positive-strand viruses.

Satsuma dwarf and related viruses belong to a new lineage of plant picorna-like viruses

Satsuma dwarf virus (SDV) and two closely related viruses, Citrus mosaic (CiMV), and Naval orange infectious mottling (NIMV), seriously affect citrus varieties grown in Japan and East Asia. All three viruses have icosahedral particles built of two proteins encapsidating two single-stranded genomic RNAs. The natural mode of transmission of these SDV-like viruses is unknown, and they were previously placed among tentative members of the family Comoviridae. Recently, a complete genome of SDV was sequenced, and its replication-related proteins were found only distantly related to those of viruses from the family Comoviridae (Iwanami T., Kondo Y., and Karasev A.V. J Gen Virol 80, 793-797, 1999). Here we present a partial genome sequence for another SDV-like virus, NIMV, and a thorough phylogenetic analysis of the gene products encoded by SDV, CiMV, and NIMV to assess their relationships with picorna-like viruses infecting plants, insects, and vertebrates. The RdRp's of SDV-like viruses form a new lineage, separate from members of Como- and Sequiviridae families. Phylogenetic analysis suggests that SDV-like viruses may represent a new family of plant picorna-like viruses. Sequence analysis of the capsid proteins (CPs) encoded by the SDV-like viruses revealed a region of similarity to CPs of animal calici- and picornaviruses that encompasses the structural core of the eight-strand beta-barrel characteristic of picornaviral CPs. These data suggest that SDV and related bipartite viruses evolved separately from the viruses in the family Comoviridae and that the split of an ancestor, monopartite picorna-like virus genome might have occurred more than once.

Nucleotide sequences and taxonomy of satsuma dwarf virus

The nucleotide sequences of genomic RNA1 (6795 nt) and RNA2 (5345 nt) of satsuma dwarf virus (SDV), a tentative member of the genus Nepovirus, were determined. The deduced genome organization of SDV showed similarities to the organization in como-, faba- and nepoviruses. There is extensive amino acid sequence similarity in the N-terminal regions of the proteins encoded by RNA1 and RNA2, as reported previously only for tomato ringspot nepovirus. However, unlike definitive nepoviruses, which have a single coat protein, SDV has two coat proteins. SDV RNA2 does not contain the long (> 1300 nt) 3' non-coding region characteristic of some nepoviruses. Phylogenetic analysis of SDV RNA polymerase placed SDV apart from como-, faba- and nepoviruses. These unique features suggest that SDV is distinct from the Comovirus, Fabavirus and Nepovirus genera, and needs to be separated into a new genus, probably within the family Comoviridae.