Is a closing GA pair a rule for stable loop-loop RNA complexes?

RNA hairpin aptamers specific for the trans-activation-responsive (TAR) RNA element of human immunodeficiency virus type 1 were identified by in vitro selection (Duconge, F., and Toulme, J. J. (1999) RNA 5, 1605-1614), The high affinity sequences selected at physiological magnesium concentration (3 mM) were shown to form a loop-loop complex with the targeted TAR RNA. The stability of this complex depends on the aptamer loop closing GA pair as characterized by preliminary electrophoretic mobility shift assays. Thermal denaturation monitored by UV-absorption spectroscopy and binding kinetics determined by surface plasmon resonance show that the GA pair is crucial for the formation of the TAR-RNA aptamer complex. Both thermal denaturation and surface plasmon resonance experiments show that any other pairs leads to complexes whose stability decreases in the order AG > GG > GU > AA > GC > UA >> CA, CU. The binding kinetics indicate that stability is controlled by the off-rate rather than by the on-rate. Comparison with the complex formed with the TAR* hairpin, a rationally designed TAR RNA ligand (Chang, K. Y,, and Tinoco, I. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 8705-8709), demonstrates that the GA pair is a key determinant which accounts for the 50-fold increased stability of the TAR-aptamer complex (K-d = 2.0 nM) over the TAR-TAR* one (K-d = 92.5 nM) at physiological concentration of magnesium. Replacement of the wild-type GC pair next to the loop of RNA I' by a GA pair stabilizes the RNA I'-RNA II' loop-loop complex derived from the one involved in the control of the ColE1 plasmid replication. Thus, the GA pair might be the preferred one for stable loop-loop interactions.