Pervasive RNA secondary structure in the genomes of SARS-CoV-2 and other coronaviruses - an endeavour to understand its biological purpose

The ultimate outcome of the COVID-19 pandemic is unknown and is dependent on a complex interplay of its pathogenicity, transmissibility and population immunity. In the current study, SARS coronavirus 2 (SARS-CoV-2) was investigated for the presence of large scale internal RNA base pairing in its genome. This property, termed genome scale ordered RNA structure (GORS) has been previously associated with host persistence in other positive-strand RNA viruses, potentially through its shielding effect on viral RNA recognition in the cell. Genomes of SARS-CoV-2 were remarkably structured, with minimum folding energy differences (MFEDs) of 15%, substantially greater than previously examined viruses such as HCV (MFED 7-9%). High MFED values were shared with all coronavirus genomes analysed created by several hundred consecutive energetically favoured stem-loops throughout the genome. In contrast to replication-association RNA structure, GORS was poorly conserved in the positions and identities of base pairing with other sarbecoviruses - even similarly positioned stem-loops in SARS-CoV-2 and SARS-CoV rarely shared homologous pairings, indicative of more rapid evolutionary change in RNA structure than in the underlying coding sequences. Sites predicted to be base-paired in SARS-CoV-2 showed substantially less sequence diversity than unpaired sites, suggesting that disruption of RNA structure by mutation imposes a fitness cost on the virus which is potentially restrictive to its longer evolution. Although functionally uncharacterised, GORS in SARS-CoV-2 and other coronaviruses represent important elements in their cellular interactions that may contribute to their persistence and transmissibility.