Prediction of pairing schemes in RNA molecules-loop contributions and energy of wobble and non-wobble pairs.

Previously published models for predicting pairing schemes in RNA molecules, when applied to tRNA, give the clover leaf structure in only half the cases. We made a systematic investigation of the predictability of the clover leaf structure under various assumptions concerning the energetic contributions of single and double-stranded regions. We tested 21 different models and variants on a set of 100 tRNA sequences and many other variants on a smaller set of sequences. In our models we allowed not only G.C, A.U and G.U pairing, but also every other pair. Under conditions which are much less restrictive than those of previous attempts, we can nevertheless reach 90 per cent predictability for the clover leaf structure of tRNA. A most surprising and far-reaching result is that we can assign to C.G and C.C pairs binding energies quite close to the energies of G.U pairs, and still predict the clover leaf. The following ranking for non-complementary pairs was obtained : G.U, G.G and C.C, U.U, C.A, A.A and G.A, U.C. The main practical innovation which made possible the improvements in predictability are: i) not counting the stacking of base pairs separated by a bulge loop; ii) making the terminal C.C's in stems more stable than the terminal A.U's by merely -- 0.7 kcal; iii) replacing the distinction between G.C and A.U-closed loops by a distinction based on the presence of loop-favoring residues; iv) carefully adjusting the energetic balance between the various kinds of loops; v) narrowing the gap between the GC/GC and the GC/AU contributions; vi) using observations on nearest-neighbours in tRNA sequences to refine the contributions of G.U pairs.