Transcription by all RNA polymerases (RNAPs) requires a series of large-scale conformational changes to form the transcriptionally competent open complex RPo. At the lambda P-R promoter, Escherichia coli sigma(70) RNAP first forms a wrapped, closed IOU bp complex I. The subsequent step opens the entire DNA bubble, creating the relatively unstable (open) complex I-2. Additional conformational changes convert I-2 to the stable RPo. Here we probe these events by dissecting the effects of Ni+ salts of Glu(-), F-, and Cl- on each step in this critical process. Rapid mixing and nitrocellulose filter binding reveal that the binding constant for I-1 at 25 degrees C is similar to 30-fold larger in Glu(-) than in Cl- at the same Na+ concentration, with the same log-log salt concentration dependence for both anions. In contrast, both the rate constant and equilibrium constant for DNA opening to 12) are only weakly dependent on salt concentration, and the opening rate constant is insensitive to replacement of Cl- with Glu(-). These very small effects of salt concentration on a process (DNA opening) that is strongly dependent on salt concentration in solution may indicate that the backbones of both DNA strands interact with polymerase throughout the process and/or that compensation is present between ion uptake and release. Replacement of Cl- with Glu(-) or F- at 25 degrees C greatly increases the lifetime of RP and greatly reduces its salt concentration dependence. By analogy to Hofmeister salt effects on protein folding, we propose that the excluded anions Glu(-) and F- drive the folding and assembly of the RNAP clamp/jaw domains in the conversion of 12 to RP, while Cl- does not. Because the Hofmeister effect of Glu(-) or largely compensates for the destabilizing Coulombic effect of any salt on the binding of this assembly to downstream promoter DNA, RPo remains long-lived even at 0.5 M Na+ in Glu(-) or F- salts. The observation that E sigma(70) RPo complexes are exceedingly long-lived at moderate to high Glu(-) concentrations argues that E sigma(70) RNAP does not dissociate from strong promoters in vivo when the cytoplasmic glutamate concentration increases during osmotic stress.
