Genome packaging is an essential housekeeping process in virtually all organisms for properstorage and maintenance of genetic information. Although the extent and mechanisms ofpackaging vary, the process involves the formation of nucleic-acid superstructures. Crystalstructures of DnA coiled coils indicate that their geometries can vary according to sequenceand/or the presence of stabilizers such as proteins or small molecules. However, suchsuperstructures have not been revealed for RnA. Here we report the crystal structure of anRnA supercoil, which displays one level higher molecular organization than previously reportedstructures of DnA coiled coils. In the presence of an RnA-binding protein, two interlockingRnA coiled coils of double-stranded RnA, a ‘coil of coiled coils’, form a plectonemic supercoil.molecular dynamics simulations suggest that protein–RnA interaction is required for thestability of the supercoiled RnA. This study provides structural insight into higher orderpackaging mechanisms of nucleic acids.
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