Table 1.
Comparison of LacI and purR functions
| LacI | PurR | Reference | ||
| DNA binding | ||||
| Matched conditionsa | Kd (M) | 7.2 × 10−9 | b+G: 1.9 × 10−10 | Xu et al. 1998 |
| c+I: n.d. | no G: 4.0 × 10−8 | |||
| kdiss (s−1) | 3.7 × 10−2 | b+G: 1.2 × 10−3 | ||
| +I: n.d. | no G: 2.8 × 10−2 | |||
| Optimal conditions | kdiss (s−1)d | 6 × 10−4 | Riggs et al. 1970b | |
| e+I: 1 × 10−1 | ||||
| Kd (M)/ | 4.3 × 10−13/ | +G: 3 × 10−12/ | Whitson 1985 | |
| [salt] (M) | 100 mM KClf | 200 mM KClg | Moraitis et al. 2001 | |
| Effector bindingb,d | ||||
| Matched conditionsa | Kd (M)/nh | 0.8 × 10−6/1 | 7.0 × 10−6/1 | Xu et al. 1998 |
| +DNA: 9.8 × 10−7/1 | ||||
| Optimal conditions | Kd (M)/nh | 4 × 10−6/1i | O'Gorman et al. 1980 | |
| +DNA: 8 × 10−5/1.5 | ||||
a Measurements were made in 100 mM HEPES-KOH, pH 7.5, 250 mM potassium glutamate, 150 mM sodium chloride, 10 mM magnesium acetate, and 1 mM EDTA. Operator DNA sequences were 30 bp purF for PurR and 40 bp O1 for LacI. Equilibrium constants for DNA binding are expressed in units of M dimer. Constants for effector binding have the units M monomer.
b G corresponds to the co-repressor guanine. PurR + G is the state with high affinity for DNA.
c I corresponds to inducer IPTG. LacI + I is the state with low affinity for DNA.
d Solution conditions were 10 mM magnesium acetate, 10 mM potassium chloride, 10 mM Tris-HCl, pH 7.4, 10−4 M EDTA, 10−4 M dithiothreitol, 5% dimethylsulfoxide, 50 μg/ml BSA. Operator was λφ80dlac (30 × 106 daltons).
e This experiment measured the DNA off-rate upon the addition of inducer and thus reflects the rate with which LacI responds to its allosteric signal.
f Experimental conditions were 10 mM Tris-HCl, pH 7.4, 10−4 M dithiothreitol, 10−4 M EDTA, 5% dimethylsulfoxide, and 50 μg/ml BSA as well as 100 mM KCl noted in the table. Operator DNA was a 40 bp fragment containing the O1 binding site. The equilibrium DNA binding constant has the units M tetramer.
g Solution conditions included 10 mM Tris-HCl, pH 7.6, 100 μg/ml BSA, and 5% DMSO as well as 200 mM KCl noted in the table. Operator DNA was 30 bp purF. The equilibrium constant for DNA binding was calculated as M dimer.
h Hill coefficient.
i Measurements performed in 10 mM Tris-HCl, 3 × 10−4 M dithiothreitol, 5% glycerol, pH 7.5. Inducer binding is not sensitive to changes in salt concentration. 40 bp operator DNA corresponding to O1 was isolated from pBR345.