Pronounced instability of tandem IU base pairs in RNA.

Optical melting was used to determine the stabilities of three series of RNA oligomers containing tandem XU base pairs, GGCXUGCC (5'XU3'), GGCUXGCC (5'UX3') and GGCXXGGC/CCGUUCCG (5'XX3'), where X is either A, G or I (inosine). The helices containing tandem AU base pairs were the most stable in the first two series (5'XU3' and 5'UX3'), with an average melting temperature approximately 11 degrees C higher than the helices with tandem 5'GU3' base pairs and 25 degrees C higher than the helices with tandem 5'IU3' base pairs. For the third series (5'XX3'), the helix containing tandem GG is the most stable, with an average melting temperature approximately 2 degrees C higher than the helix with tandem AA base pairs and approximately 24 degrees C higher than the helix with tandem II base pairs. The thermodynamic stability of the oligomers with tandem IU base pairs was also investigated as a function of magnesium ion concentration. As with normal A-U or G-U tandem duplexes, the data could best be interpreted as non-specific binding of magnesium ions to the inosine-containing RNA oligonucleotides.