Cucumber mosaic cucumovirus (CMV) can be divided into two subgroups, I and II. LS-CMV and most other subgroup II strains cause a mild, systemic mottle on tobacco and can induce a necrotic etching (necrotic rings) symptom on inoculated tobacco leaves. In contrast, Fny-CMV and most other subgroup I strains cause severe, systemic mosaic symptoms on tobacco, but do not induce the necrotic etching symptom. Full-length cDNA clones of all three genomic RNAs of LS-CMV were constructed and infectious RNAs were generated from these clones. Using pseudorecombinants constructed from the infectious transcripts of LS-CMV and Fny-CMV, we found that both RNAs 1 and 2 of Fny-CMV are involved in determining the severity of systemic symptom on tobacco, and that LS-CMV RNA 3 contains the determinant for the necrotic etching symptom. Chimeras formed between Fny- and LS-CMV RNA 3 were used to demonstrate that the inducer of the necrotic etching symptoms mapped to the 5′ 618 nucleotides of LS-CMV RNA 3, and required sequences in both the 5′ non-translated region, as well as the 3a gene of CMV.


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