Crystal structures of a series of RNA aptamers complexed to the same protein target.

We have determined the crystal structures, at 2.8 A resolution, of two different RNA aptamers, each bound to MS2 coat protein. One of the aptamers contains a non-Watson-Crick base pair, while the other is missing one of the unpaired adenines that make sequence-specific contacts in the wild-type complex. Despite these differences, the RNA aptamers bind in the same location on the protein as the wild-type translational operator. Comparison of these new structures with other MS2-RNA complexes allows us to refine further the definition of the minimal recognition elements and suggests a possible application of the MS2 system for routine structure determination of small nucleic acid motifs.