Structural heterogeneity and dynamics in the apical stem loop of s2m from SARS-CoV-2 Delta by an integrative NMR spectroscopy and MD simulation approach

In structured RNAs, helical elements are often capped by apical loops that are integral structural elements, ranging from 3 to >20 nts of size on average, and display a highly heterogeneous energy landscape profile, rendering structural characterization particularly challenging. We here provide a characterization of the SARS-CoV-2 Delta s2m element containing a highly dynamic nonaloop using an integrative approach of nuclear magnetic resonance spectroscopy (NMR), small angle X-ray scattering (SAXS), and molecular dynamics simulations (MD). We further explored the conformational space in the s2m nonaloop and its transient closing 5'-G-U-3' base pair by MD simulations weighted by experimental NMR observables, leading to a comprehensive representation of the s2m nonaloop motif. Our deconvolution of the ensemble into conformations and dynamics provides a basis for future ensemble-functional characterization of RNA structures featuring dynamic motifs.