Umbravirus-encoded proteins both stabilize heterologous viral RNA and mediate its systemic movement in some plant species.

The proteins encoded by open reading frame 3 (ORF3) of the umbraviruses pea enation mosaic virus-2 and tobacco mottle virus, like that of groundnut rosette virus, mediated the movement of viral RNA through the phloem of infected Nicotiana benthamiana or N. clevelandii plants when they were expressed from chimeric tobacco mosaic virus in place of the coat protein. However, these chimeras did not move systemically in N. tabacum. In lysates of N. benthamiana or N. tabacum protoplasts, the chimeric RNAs were more stable than was RNA of tobacco mosaic virus lacking the coat protein gene. The chimeric viruses also protected the latter in trans, suggesting that the ORF3 proteins can increase the stability of heterologous viral RNA. Umbraviral ORF3 proteins contain a conserved arginine-rich domain, and the possible roles of this motif in the functions of the proteins are discussed.