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 (C iMV), 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 R NAs. The natural mode of transmission of these SDV-like viruses is unknown, and they were previously placed among tentative members of the family Como viridae. Recently, a complete genome of SDV was sequenced, and its replicat ion-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 V irol 80, 793-797, 1999). Here we present a partial genome sequence for anot her 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 viruse s may represent a new family of plant picorna-like viruses. Sequence analys is of the capsid proteins (CPs) encoded by the SDV-like viruses revealed a region of similarity to CPs of animal calici- and picornaviruses that encom passes the structural core of the eight-strand beta -barrel characteristic of picornaviral CPs. These data suggest that SDV and related bipartite viru ses 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.