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A cucumber mosaic virus (CMV-Ix) from Ixora is unusual in that it does not support the accumulation of some well-characterized CMV satellite RNAs in plants. CMV-Ix can support a particular satellite RNA variant which causes lethal tomato necrosis when inoculated with other CMV strains but not when inoculated with CMV-Ix. This difference in ability to support accumulation of specific satellite variants is apparent even when their sequences differ by only 10 nucleotides. Electroporation of tomato protoplasts with combinations of CMV-Ix or CMV-1 RNA plus the same satellite variants showed similar differences in accumulation, indicating a defect in satellite RNA replication and not movement or encapsidation. Pseudorecombinant virus infections between CMV-1 and CMV-Ix indicated that the genomic determinants responsible for this phenotype reside on RNA 1 since only combinations with CMV-Ix RNA 1 failed to replicate satellite RNA. The complete genome of CMV-Ix was cloned, sequenced and compared with the genomes of other cucumoviruses. CMV-Ix is most similar in RNA and protein sequence to subgroup 1 CMV-Fny and CMV-Y but slightly less similar than they are to each other. CMV-Ix and all cucumovirus strains sequenced thus far share a domain in the 3' untranslated portion of their genomic RNAs in which 39 of 40 bases are completely conserved.