Modified nucleoside-dependent transition metal binding to DNA analogs of the tRNA anticodon stem/loop domain

Biologically active DNA analogs of tRNA^Phe (tDNA^Phe) were used to investigate metal ion interaction with tRNA-like structures lacking the 2OH. Binding of Mg²⁺ to the 76 oligonucleotide tDNA^Phe, monitored by circular dichroism spectroscopy, increased base stacking and thus the conformational stability of the molecule. Mg²⁺ binding was dependent on a d(m⁵C) in the anticodon region. In contrast to Mg²⁺, Cd²⁺ decreased base stacking interactions, thereby destabilizing the molecule. Since alterations in the anticodon region contributed to most of the spectral changes observed, detailed studies were conducted with anticodon hairpin heptadecamers (tDNA_AC ^Phe). The conformation of tDNA_AC ^Phe-d(m⁵C) in the presence of 1 mm Cd²⁺, Co²⁺, Cr²⁺, Cu²⁺, Ni²⁺, Pb²⁺, VO²⁺ or Zn²⁺ differed significantly from that of the biologically active structure resulting from interaction with Mg²⁺, Mn²⁺ or Ca²⁺. Nanomolar concentrations of the transition metals were sufficient to denature the tDNA_AC ^Phe-d(m⁵C) structure without catalyzing cleavage of the oligonucleotide. In the absence of Mg²⁺ and at Cd²⁺] to tDNA_Ac ^Phe-d(m⁵C)] ratios of approximately 0.2–1.0, tDNA_AC ^Phe-d(m⁵C₄₀) formed a stable conformation with one Cd²⁺ bound with a K _d = 3.7 × 10^-7. In contrast to Mg²⁺, Cd²⁺ altered the DNA analogs without discriminating between modified and unmodified tDNA_AC ^Phe. This ability of transition metals to disrupt higher order DNA structures, and possibly RNA, at M concentrations, in vitro, demonstrates that these structures are potential targets in chronic metal exposure, in vivo.