Interactions of poly-N6-methyladenylic Acid and N6-substituted-9-methyladenines with poly-5-bromouridylic Acid: A Novel Base-pairing

Abstract

The interaction of poly-N⁶-methyladenylic acid (poly(m⁶A)) with poly-5-bromouridylic acid (poly(BU)) was studied by the mixing curve method. A 1 m⁶A: 2 BU stoichiometry was clearly indicated over a wide range of ionic strengths at neutral pH, while the binding of poly(m⁶A) to poly(U) is known to occur with 1 m⁶A:1 U. Digestion by nuclease S1 confirmed this stoichiometry, indicating the absence of single strands in a 1:2 mixture. Heating profile analysis and hydroxyapatite column chromatography provided further confirmation of this finding. To determine whether 1:2 stoichiometry holds in a monomer-polymer system, the interaction of N⁶-methyl-9-methyladenine (m⁶m⁹A), a corresponding monomer of poly(m⁶A), with poly(BU) was investigated.

Equilibrium dialysis experiments showed the stoichiometry of the interaction to be 1 m⁶A: 2 BU. Thus, we would describe some structural studies of the above complexes using c.d. and i. r. spectroscopy. Poly (m⁶A)·2poly(BU) and m⁶m⁹A·2poly(BU) are helical and analogous to each other in structure, and the bases in the complexes are all bound by hydrogen-bonding. N⁶-(Δ²-isopentenyl)- and N⁶-allyl-9-methyladenine were also found to form complexes with poly(BU), giving similar c.d. spectra with that of m⁶m⁹A·2poly(BU). The melting experiments indicated the T_ms to be substantially decreased, compared to the parent unmodified complexes, even though the T_m dependence of the polymer complex on salt concentration conforms to the typical triple strand. In the following, the biological significance of this novel pairing will be discussed.