The Structure And Function Of Cucumber Mosaic Virus Satellite RNA

Cucumber Mosaic virus(CMV)carrying satellite RNA(satRNA)is a common biological phenomenon.Presence of satRNAs usually reduces CMV accumulation and alleviates symptom expression.SatRNA is a non-coding RNA,whose biological activity must rely on its structure.Nevertheless,it is still known poorly about the molecular mechanisms of the attenuated functional domains of satRNAs.Moreover,secondary structure directly related to the biological phenotype has not been identified yet.We found a CMV-associated satRNA(sat-T1)that ameliorated CMV-induced symptoms,accompanied with a significant reduction in the accumulation of viral genomic RNAs 1 and 2,which encode the viral replicase.In trans replication assays indicate that the reduced accumulation is the outcome of replication competition.Via RNA structure prediction and functional analysis,the structural basis for the competition was delineated to the 3' side hairpin in a three way-branched secondary structure that locates downstream of the center of sat-T1.The three way-branched secondary structure contains two biologically important hairpins.One is indispensable for the competition,and the other engages in formation of a tertiary pseudoknot structure that is essential for sat-T1 survival.The three way-branched structure with the pseudoknot is designated y-shaped structure(ySS),which is the first substructure with an important biological relevance identified in CMV satRNAs so far.We propose a model in which CMV satRNAs reduce viral accumulation and symptom expression by outcompeting replicase-encoding RNAs of the helper virus for replication-associated host factor(s)via the newly-identified structure.In a summary,we reported the secondary structure and pseudoknot with biological significance in CMV satRNAs.Meanwhile,we uncovered the structural basis responsible for the downregualtion of satRNAs on virus accumulation and symptom expression.Our findings allow us to propose a model in which CMV satRNAs downregulate viral accumulation and symptom expression.Our work provided a foundation for further investigation of replication competition between satRNAs and helper viruses.