RNA FOLDING CAUSES SECONDARY STRUCTURE REARRANGEMENT

The secondary structure of the P5abc subdomain (a 56-nt RNA) of the Te trahymena thermophila group I intron ribozyme has been determined by N MR, Its base pairing in aqueous solution in the absence of magnesium i ons is significantly different from the RNA in a crystal but is consis tent with thermodynamic predictions. On addition of magnesium ions, th e RNA folds into a tertiary structure with greatly changed base pairin g consistent with the crystal structure: three Watson-Crick base pairs , three G.U base pairs, and an extra-stable tetraloop are lost. The co mmon assumption that RNA folds by first forming secondary structure an d then forming tertiary interactions from the unpaired bases is not al ways correct.