Figure 4. The Effect of Cd²⁺ on Activity of Phosphorothioate-Containing Ribozymes.

Reactions contained saturating G and ribozyme, and monitored cleavage of −1d,rSA₅ (see Table 1) in the presence or absence of 0.1 mM Cd²⁺ (10 mM Mg²⁺ background). The largest Cd²⁺ stimulation occurred with the C262-S _P variant ribozyme. Significant Cd²⁺ stimulation remained unique to this ribozyme at all Cd²⁺ concentrations tested (0.1–10 mM; data not shown). Concentrations of Cd²⁺ above 1 mM inhibit the ribozyme reaction with a steep concentration dependence (see, e.g., Figures S4 and S5A). The small Cd²⁺ stimulation observed for the U258 variant ribozymes is consistent with coordination of a metal ion important for structural stability, as proposed by Lindqvist et al. [78].