Purine-rich regions in DNA and RNA may contain both guanines and adenines, which have various biological functions. Here we report the crystal structure of an RNA purine-rich fragment containing both guanine and adenine at 1.4 A resolution. Adenines form an adenine tetrad in the N6-H em leader N7 conformation. Substitution of an adenine tetrad in the guanine tetraplex does not change the global conformation but introduces irregularity in both the hydrogen bonding interaction pattern in the groove and the metal ion binding pattern in the central cavity of the tetraplex. The irregularity in groove binding may be critical for specific binding in tetraplexes. The formation of G-U octads provides a mechanism for interaction in the groove. Ba(2+) ions prefer to bind guanine tetrads, and adenine tetrads can only be bound by Na(+) ions, illustrating the binding selectivity of metal ions for the tetraplex.