Satellite RNA of Bamboo mosaic virus (satBaMV), a single-stranded mRNA type satellite encoding a protein of 20 kDa (P20), depends on the helper BaMV for replication and encapsidation. Two satBaMV isolates, BSF4 and BSL6, exhibit distinctly differential phenotypes in Nicotiana benthamiana plants when coinoculated with BaMV RNA. BSL6 significantly reduces BaMV RNA replication and suppresses the BaMV-induced symptoms, whereas BSF4 does not. By studies with chimeric satBaMVs generated by exchanging the components between BSF4 and BSL6, the genetic determinants responsible for the downregulation of BaMV replication and symptom expression were mapped at the 5 untranslated region (UTR) of BSL6. The 5 UTR of BSL6 alone is sufficient to diminish BaMV RNA replication when the 5 UTR is inserted in cis into the BaMV expression vector or when coinoculation with mutants that block the synthesis of P20 protein takes place. Further, the 5 UTR of natural satBaMV isolates contains one hypervariable (HV) region which folds into a conserved apical hairpin stem-loop ( Satellite RNAs (satRNAs) depend on their helper viruses for replication, encapsidation, and systemic movement, but they share little or no sequence similarity with the helper virus genome (reviewed in reference 38). Replication of satRNA can interfere with helper virus replication and modify the symptoms induced by the helper virus. The molecular interactions depend on the host plants, the strain of helper viruses, and the satRNA (6). For instance, some cucumber mosaic virus (CMV) satRNAs could attenuate the symptoms induced by CMV in tomato plants and others could intensify chlorotic symptoms in tomato and tobacco or necrosis in tomato (6). Sequences specifying chlorosis or necrosis were delimited to the 5Ј and/or 3Ј half of the satRNA (11, 29). The 5Ј half of minus-stranded satRNAs could induce tomato necrosis, the necrogenicity domain located in an octanucleotide loop and adjacent stem of a hairpin structure (46). Another example is the satRNA C of Turnip crinkle virus (TCV) either intensifying or attenuating symptoms, depending on the host and the level of coat protein (CP) in TCV-infected plants (16,19,39,53). Since the CP of TCV is a silencing suppressor, satRNA C can reduce the encapsidation of genomic RNA and increase the level of free CP, which implies that satRNA C may enhance TCV to overcome posttranscriptional gene silencing (47).In many cases, attenuation of symptoms is usually accompanied by a reduction in the helper virus titer (38). The attenuation associated with CMV satRNA is due to competition with the helper virus for replication by RNA-dependent RNA polymerase (RdRp) (13, 55). However, when tomato aspermy cucumovirus (TAV) was the helper virus, some satRNAs could attenuate the TAV-induced symptoms but not the level of TAV RNAs (31). Evidence also suggests that subviral RNAs can enhance the resistance of host plants (40).Bamboo mosaic virus (BaMV), a member of the potexvirus group, contains a single-stranded positive-sense RNA genome with five conse...

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