Biologically-supported structural model for a viral satellite RNA.

Satellite RNAs (satRNAs) are a class of small parasitic RNA replicon that associate with different viruses, including plus-strand RNA viruses. Because satRNAs do not encode a polymerase or capsid subunit, they rely on a companion virus to provide these proteins for their RNA replication and packaging. SatRNAs recruit these and other required factors via their RNA sequences and structures. Here, through a combination of chemical probing analysis of RNA structure, phylogenetic structural comparisons, and viability assays of satRNA mutants in infected cells, the biological importance of a deduced higher-order structure for a 619 nt long tombusvirus satRNA was assessed. Functionally-relevant secondary and tertiary RNA structures were identified throughout the length of the satRNA. Notably, a 3'-terminal segment was found to adopt two mutually-exclusive RNA secondary structures, both of which were required for efficient satRNA accumulation. Accordingly, these alternative conformations likely function as a type of RNA switch. The RNA switch was also found to engage in a required long-range kissing-loop interaction with an upstream sequence. Collectively, these results establish a high level of conformational complexity within this small parasitic RNA and provide a valuable structural framework for detailed mechanistic studies.