Table 2.

Effect of Mg²⁺ concentration on RNA-dependent ATPase activity of Mss116p

	kobs(min−1)
Mg2+ (mM)	aI5γ	bI1	poly (C)
1	122 ± 18	64 ± 16	10 ± 10
3	90 ± 22	58 ± 12	60 ± 8
5	88 ± 16	78 ± 12	60 ± 12
8	112 ± 6	110 ± 14	62 ± 12
10	84 ± 18	80 ± 14	58 ± 4
15	68 ± 22	62 ± 18	36 ± 12
25	82 ± 12	56 ± 6	44 ± 12
35	34 ± 3	50 ± 12	48 ± 14
50	32 ± 4	28 ± 6	46 ± 6

ATPase activity was assayed with 50 nM Mss116p in reaction medium containing 100 mM KCl and different Mg²⁺ concentrations at 30°C, in the presence of 3 mM ATP and 300 nM intron RNAs or 0.25 μg/μl poly(C). The RNA concentrations are saturating in reaction medium containing 100 mM KCl and 8 mM Mg²⁺. k_obs values were calculated from initial rates in time courses (see Table 1). In the absence of RNA, ATPase activity was not detectable at any of the Mg²⁺ concentrations (not shown). Values are the mean ± the standard deviation for triplicate assays.