Table 1. k_obs for cleavage of VS substrates by ribozyme variants under various ionic conditions.

k/k′ corresponds to the ratio $(k_o^{Mut}/k_o^{WT})$, where the subscript o corresponds to the oxo substrate, and superscripts Mut, WT correspond to a ribozyme mutant and wildtype, respectively. Metal ion rescue is calculated according to the ratio of the ratios (k_o/k_s)^Mg2+/(k_o/k_s)^Cd2+, where the subscript s corresponds to the thio substrate, superscripts Mg²⁺ and Cd²⁺ correspond to experiments in presence of Mg²⁺ alone, and experiments in presence of both Mg²⁺ and Cd²⁺, respectively. Mutational rescue corresponds to the ratio of the ratios (k_o/k_s)^WT/(k_o/k_s)^Mut. The error of k_obs measurements is the standard deviation of three or more independent measurements and errors for thio effect, metal ion rescue, and mutational rescue are the propagated errors of relative errors. Reactions that proceed with slower than the limit of detection (<5% cleavage after 24 hours) have been assigned a rate of 10⁻⁴ min⁻¹ and cannot be assigned an error (“n.a.”). Propagated errors as well as ratios based on these measurements are also “n.a.”

variant	ionic conditions	substrate	kobs (min−1)	k/k′	ko/ks	metal ion rescue	mutational rescue
Wild Type	10mM Mg2+	oxo	0.32 ± 0.01
		A621 SP	< 0.0001		> 3.2 × 103
		A621 RP	< 0.0001		> 3.2 × 103
		A622 SP	0.0294 ± 0.0008		11 ± 0.3
		A622 RP	< 0.0001		> 3.2 × 103
	10mM Mg2+	oxo	0.283 ± 0.005
	and	A621 SP	0.0053 ± 0.0001		53 ± 1.4	> 60 ± n.a.
	20μM Cd2+	A621 RP	< 0.0001		> 2.8 × 103	n.a.
		A622 SP	0.0328 ± 0.003		8.6 ± 0.8	1.3 ± 0.1
		A622 RP	0.0190 ± 0.007		15 ± 5.5	> 213 ± n.a.
G638I	10mM Mg2+	oxo	0.0135 ± 0.0006	24 ± 1.3
		A621 SP	< 0.0001		> 130		n.a.
		A621 RP	0.0020 ± 0.0001		6.8 ± 0.5		> 470 ± n.a.
	10mM Mg2+	oxo	0.0141 ± 0.0005	21 ± 0.8
	and	A621 SP	0.0009 ± 0.0001		16 ± 1.8	> 8.1 ± n.a.	3.3 ± 0.1
	20μM Cd2+	A621 RP	0.0023 ± 0.0001		6.1 ± 0.3	1.1 ± 0.1	> 460 ± n.a.
A756(3cP)	10mM Mg2+	oxo	0.0336 ± 0.0002	9.5 ± 0.3
		A621 SP	< 0.0001		> 330		n.a.
		A621 RP	0.0041 ± 0.0001		8.2 ± 0.2		> 390 ± n.a.
	10mM Mg2+	oxo	0.035 ± 0.002	8.1 ± 0.5
	and	A621 SP	0.0016 ± 0.0001		22 ± 1.9	> 15 ± n.a.	2.4 ± 0.2
	20μM Cd2+	A621 RP	0.0033 ± 0.0001		11 ± 0.7	0.8 ± 0.1	> 250 ± n.a
G638I	10mM Mg2+	oxo	0.0023 ± 0.0004	140 ± 7.4
and		A621 SP	< 0.0001		> 23		n.a.
A756(3cP)		A621 RP	0.0140 ± 0.0006		0.2 ± 0.1		> 16 × 103 ± n.a.
	10mM Mg2+	oxo	0.0018 ± 0.0001	160 ± 9.2
	and	A621 SP	0.0009 ± 0.0005		2 ± 1.1	> 12 ± n.a.	27 ± 15
	20μM Cd2+	A621 RP	0.011 ± 0.003		0.2 ± 0.1	1.0 ± 0.5	> 14 × 103 ± n.a.