Disorder and twin refinement of RNA heptamer double helices

An RNA helix with seven base pairs which was derived from the acceptor stem of Escherichia coli tRNA(Ala) rGGGGCUA.rUAGCUGC (ALA(wt)), as well as a va riant, rGGGGCUA.rUAGCCCC (ALA(C70)), in which the single G.U wobble base pa ir of ALA(wt) was replaced by G.C, crystallize in space group C2. Both non- isomorphic crystal forms display a complex packing pattern, which can be de scribed alternatively as disorder or pseudo-merohedral twinning. The struct ure of ALA(wt) was determined by SIRAS phasing using an isomorphous iodine derivative, rGGGGCi(5)UA.rUAGCUCC (ALA(I)). All three RNA structures were s ubsequently subjected to twin refinement in space group P1, using anisotrop ic thermal displacement parameters at resolutions of 1.16, 1.23 and 1.4 Ang strom for ALA(wt), ALA(I) and ALA(C70), respectively. Alternatively, the st ructure of ALA(wt) was refined in space group C2 assuming twofold disorder of the molecular orientation, The refined structures are of reasonable qual ity according to all available indicators. There are no systematic differen ces between the molecular models resulting from twin refinement and disorde r refinement.