Most deoxyribozymes (DNA catalysts) require metal ions as cofactors for catalytic activity, with Mg2+, Mn2+, and Zn2+ being the most represented activators. Trivalent transition-metal ions have been less frequently considered. Rare earth ions offer attractive properties for studying metal ion binding by biochemical and spectroscopic methods. Here we report the effect of lanthanide cofactors, in particular terbium (Tb3+), for DNA-catalyzed synthesis of 2',5'-branched RNA. We found up to 10(4)-fold increased ligation rates for the 9F7 deoxribozyme using 100 mu M Tb3+ and 7 mM Mg2+, compared to performing the reaction with 7 mM Mg2+ alone. Combinatorial mutation interference analysis (CoMA) was used to identify nucleotides in the catalytic region of 9F7 that are essential for ligation activity with different metal ion combinations. A minimized version of the DNA enzyme sustained high levels of Tb3+-assisted activity. Sensitized luminescence of Tb3+ bound to DNA in combination with DMS probing and DNase I footprinting results supported the CoMA data. The accelerating effect of Tb3+ was confirmed for related RNA-ligating deoxyribozymes, pointing toward favorable activation of internal 2'-OH nucleophiles. The results of this study offer fundamental insights into nucleotide requirements for DNA-catalyzed RNA ligation and will be beneficial for practical applications that utilize 2',5'-branched RNA.