  Background. Dramatic increases in RNA structural data have made it possible
to recognize its conformational preferences much better than a decade ago. This
has created an opportunity to use discrete restraint-based conformational
sampling for modelling RNA and automating its crystallographic refinement.
Results. All-atom sampling of entire RNA chains, termini and loops is achieved
using the Richardson RNA backbone rotamer library and an unbiased distribution
for glycosidic dihedral angle. Sampling behaviour of Rappertk on a diverse
dataset of RNA chains under varying spatial restraints is benchmarked. The
iterative composite crystallographic refinement protocol developed here is
demonstrated to outperform CNS-only refinement on parts of tRNA(Asp) structure.
Conclusion. This work opens exciting possibilities for further work in RNA
modelling and crystallography.
