Roles of Mg2+ in the mechanism of formation and dissociation of open complexes between Escherichia coli RNA polymerase and the lambda PR promoter: kinetic evidence for a second open complex requiring Mg2+.

Comparative studies of the effects of Mg2+ vs Na+ and of acetate (OAc-) vs Cl- on the kinetics of formation and dissociation of E. coli RNA polymerase (E sigma 70)-lambda PR promoter open complexes have been used to probe the mechanism of this interaction. Composite second-order association rate constants ka and first-order dissociation rate constants kd, and their power dependences on salt concentration SKa (SKa identical to d log ka/d log [salt]) and Skd (Skd identical to d log kd/d log [salt]), were determined in MgCl2 and NaOAc to compare with the results of Roe and Record (1985) in NaCl. Replacement of NaCl by MgCl2 reduces the magnitude of Ska 2-fold (Ska = -11.9 +/- 1.1 in NaCl; Ska = -5.2 +/- 0.3 in MgCl2) and (by extrapolation) drastically reduces the magnitude of ka at any specified salt concentration (e.g., approximately 10(6)-fold at 0.2 M). Replacement of NaCl by NaOAc does not significantly affect Ska (Ska = -12.0 +/- 0.7 in NaOAc) and (by extrapolation) increased ka by approximately 80-fold at any fixed [Na+]. In the absence of Mg2+, replacement of NaCl by NaOAc is found to increase the half-life of the open complex by approximately 560-fold at fixed [Na+] without affecting Skd [Skd = 7.6 +/- 0.1 in NaOAc; in NaCl, Skd = 7.7 +/- 0.2 (Roe & Record, 1985)]. Replacement of NaCl by MgCl2 drastically reduces both Skd and the half-life of the open complex at any salt concentration below approximately 0.2 M. Strikingly, Skd = 0.4 +/- 0.1 in MgCl2, indicating that the net uptake of Mg2+ ions in the kinetically significant steps in dissociation of the open complex is much smaller than that expected by analogy with the uptake of approximately 8 Na+ ions in the corresponding steps in NaCl. In NaCl/MgCl2 mixtures, at a constant [NaCl] in the range 0.1-0.2 M, initial addition of MgCl2 (0.5 mM less than or equal to [MgCl2] less than or equal to 1 mM) increases the half-life of the open complex; further addition of MgCl2 causes the half-life to decrease, though the effect of [MgCl2] on kd is always less than that predicted by a simple competitive model. The observed effects of MgCl2 on Skd and kd differ profoundly from those expected from the behavior of kd and Skd in NaCl and NaOAc and indicate that the role of Mg2+ in dissociation is not merely that of a nonspecific divalent competitor with RNAP for interactions with DNA phosphates and of a DNA helix-stabilizer, both of which should cause kd to increase monotonically with increasing [Mg2+].(ABSTRACT TRUNCATED AT 400 WORDS)