Impact of static and dynamic A-form heterogeneity on the determination of RNA global structural dynamics using NMR residual dipolar couplings.

We examined how static and dynamic deviations from the idealized A-form helix propagate into errors in the principal order tensor parameters determined using residual dipolar couplings (rdcs). A 20-ns molecular dynamics (MD) simulation of the HIV-1 transactivation response element (TAR) RNA together with a survey of spin relaxation studies of RNA dynamics reveals that pico-to-nanosecond local motions in non-terminal Watson-Crick base-pairs will uniformly attenuate base and sugar one bond rdcs by approximately 7%. Gaussian distributions were generated for base and sugar torsion angles through statistical comparison of 40 RNA X-ray structures solved to <3.0 A resolution. For a typical number (>or=11) of one bond C-H base and sugar rdcs, these structural deviations together with rdc uncertainty (1.5 Hz) lead to average errors in the magnitude and orientation of the principal axis of order that are <9% and <4 degrees, respectively. The errors decrease to <5% and <4 degrees for >or=17 rdcs. A protocol that allows for estimation of error in A-form order tensors due to both angular deviations and rdc uncertainty (Aform-RDC) is validated using theoretical simulations and used to analyze rdcs measured previously in TAR in the free state and bound to four distinct ligands. Results confirm earlier findings that the two TAR helices undergo large changes in both their mean relative orientation and dynamics upon binding to different targets.