Mg2+‐modulated KMnO4 reactivity of thymines in the open transcription complex reflects variation in the negative electrostatic potential along the separated DNA strands

Abstract

There is still a controversy over the mechanism of promoter DNA strand separation upon open transcription complex (RPo) formation by Escherichia coli RNA polymerase: is it a single or a stepwise process controlled by Mg2+ ions and temperature? To resolve this question, the kinetics of pseudo-first-order oxidation of thymine residues by KMnO4 in the -11 ... +2 DNA region of RPo at the lambdaP(R) promoter was examined under single-hit conditions as a function of temperature (13-37 degrees C) in the absence or presence of 10 mm MgCl2. The reaction was also studied with respect to thymidine and its nucleotides (TMP, TTP and TpT) as a function of temperature and [MgCl2]. The kinetic parameters, (ox)k and (ox)E(a), and Mg-induced enhancement of (ox)k proved to be of the same order of magnitude for RPo-lambdaP(R) and the nucleotides. Unlike the complex, (ox)E(a) for the nucleotides was found to be Mg-independent. The isothermal increase in (ox)k with increasing [Mg2+] was thus interpreted in terms of a simple model of screening of the negative charges on phosphate groups by Mg2+ ions, lowering the electrostatic barrier to the diffusion of MnO4- anions to the reactive double bond of thymine. Similar screening isotherms were determined for the oxidation of two groups of thymines in RPo at a consensus-like Pa promoter, differing in the magnitude of the Mg effect. Together, the findings show that: (a) the two DNA strands in the -11...+2 region of RPo-lambdaP(R) are completely separated over the whole range of temperatures investigated (13-37 degrees C) in the absence of Mg2+ (b) Mg2+ ions induce an increase in the rate of the oxidation reaction by screening negatively charged phosphate and carboxylate groups; and (c) the observed thymine reactivity and the magnitude of the Mg effect reflect variation in the strength of the electrostatic potential along the separated DNA strands, in agreement with the current structural model of RPo.