The complete nucleotide sequence of red clover necrotic mosaic virus (RCNMV) RNA-1 has been determined. RNA-1 is 3889 nucleotides in length with a 5′ terminal m7GpppA cap. The RNA contains three large open reading frames (ORFs): the 5′ proximal ORF, encoding a 27-kDa polypeptide; the internal ORF, coding for a 57-kDa polypeptide; and the 3′ terminal ORF, encoding the 37-kDa capsid protein. The sequence results confirm in vitro translation of 27-, 50-, and 37-kDa products but do not account for the observed 90-kDa product. A translational frameshift event from the 27- to the 57-kDa ORFs is proposed to explain the synthesis of the observed 90-kDa in vitro product. The putative translational frameshift region is structurally similar to several retrovirus frameshift regions and the putative barley yellow dwarf virus (BYDV) frameshift region. Extensive amino acid homology was observed in the 57-kDa downstream ORF with the downstream domains of the carnation mottle virus (CarMV), turnip crinkle virus (TCV), maize chlorotic mottle virus (MCMV) readthrough, and BYDV fusion proteins. The 57-kDa ORF contained the conserved "GDD" motif. A significant alignment between the capsid proteins of RCNMV, CarMV, and TCV was also observed. Given the extensive amino acid sequence similarity of RCNMV, CarMV, and TCV polymerase and capsid proteins, we speculate that they are closely related, evolutionarily.
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