Table 2.

Mechanistic relationship between N4 and N117 histidine identity determinants in histidylation of resected RNA pseudoknots

RNAs, duplex no.	Ldm, experimental	Lsm1	Lsm2	Ldm, calculated	R
1 G117 C4	1	—	—	—	—
10 A117 U4	26	3	6	18	1.4
9 A117 A4 (wt)	16	3	3	9	1.8
14 A117 G4	100	3	40	120	0.8
8 U117 U4	7	2	6	12	0.6
13 U117 A4	83	2	3	6	13.8
12 U117 G4	73	2	40	80	0.9
11 C117 U4	52	35	6	20	0.25
15 C117 A4	120	35	3	105	1.1
16 C117 G4	270	35	40	1400	0.2

Activities of double-mutants are compared to that of the most active HisRS substrate, namely duplex 1. Only data obtained with closed circles are displayed. L_dm are losses of catalytic efficiency for double mutants, L_sm for single-mutants, and L_{dm(calculated}, = L_sm1 × L_sm2. Experimental data are taken from Table 1. The R factors inform about additivity, cooperativity and anticooperativity (see Materials and Methods). Notice the slight anticooperative nature of duplex 16 (R = 0.2), that disappears when the cofactor circle is open (data not shown).