Sequence Analysis and Genome Organisation of Poinsettia Mosaic Virus (PnMV) Reveal Closer Relationship to Marafiviruses Than to Tymoviruses

Abstract

Sequence comparison and genome organisation of poinsettia mosaic virus (PnMV), a putative member of the tymoviruses, revealed a closer relationship to marafiviruses. The complete nucleotide sequence of PnMV was determined. The 6099-nt RNA genome encodes a putative 221-kDa polyprotein that lacks a stop codon between the replicase and the coat protein genes, as in most tymovirus RNAs. The genomic RNA has a poly(A) tail at its 3′-terminus in contrast to the tRNA-like structure found in the RNA of most tymoviruses, and no homology was observed to the conserved noncoding region of the tymoviral 3′-termini. The tymobox of PnMV, a 16-nt region of the subgenomic RNA (sgRNA) promoter shared by most tymoviruses, differs in 3 nt from the RNA sequence of tymoviruses but is identical to the sequence of marafiviruses. At least three sgRNAs were found in PnMV-infected Euphorbia pulcherrima and in isolated PnMV particles; one that is 650 nt long encodes the 21.4-kDa coat protein, and the others are about 3.5 and 1.7 kb and contain the 5′- and the 3′-terminal parts of genomic RNA, respectively. Like tymoviruses, PnMV particles sediment as top and bottom components. The particles of the top component contain the sgRNA (650 nt) encoding the coat protein, and those of bottom component contain both genomic and sgRNAs.