Roe J H, Record M T
Biochemistry. 1985 Aug 27;24(18):4721-6. doi: 10.1021/bi00339a002.
The rate of formation of transcriptionally competent open complexes between Escherichia coli RNA polymerase (RNAP) and the lambda PR promoter is extraordinarily sensitive to the nature and concentration of the electrolyte ions in the solution. The pseudo-first-order time constant of open complex formation tau obsd, determined in excess RNAP at 25 degrees C as a function of NaCl concentration, is proportional to the concentration product [Na+]12 [RNAP]-1. Consequently, tau obsd is far more sensitive to changes in the salt concentration than to changes in the concentration of RNAP. The origin of this effect is the release of the thermodynamic equivalent of 12 monovalent ions in the process of closed complex formation at the lambda PR promoter. In more complex ionic mixtures, ion-specific stoichiometric effects on tau obsd are observed. These are not ionic strength effects but are instead both valence and species specific. Both the association and dissociation rate constants of RNAP at the lambda PR promoter are strongly salt dependent, varying (in NaCl) as [Na+]-12 and [Na+]8, respectively. Consequently, the equilibrium constant characterizing open complex formation at this promoter varies with [Na+]-20. Electrostatic interactions and counterion release are the major contributors to the binding free energy driving open complex formation in a dilute salt solution. Since the in vivo ionic environment of E. coli (and other cells) is highly variable, these large salt effects are almost certainly of physiological significance. Variations in the intracellular concentrations of inorganic and organic ions, including polyamines, must exert both global and also promoter-specific regulatory effects on the initiation of transcription, as well as on numerous other protein-nucleic acid interactions.
大肠杆菌RNA聚合酶（RNAP）与λPR启动子之间转录活性开放复合物的形成速率对溶液中电解质离子的性质和浓度极为敏感。在25℃下，于过量RNAP中测定的开放复合物形成的伪一级时间常数τobsd作为NaCl浓度的函数，与浓度乘积[Na⁺]¹²[RNAP]⁻¹成正比。因此，τobsd对盐浓度变化的敏感度远高于对RNAP浓度变化的敏感度。这种效应的根源在于在λPR启动子处形成封闭复合物的过程中释放出相当于12个单价离子的热力学量。在更复杂的离子混合物中，观察到离子特异性化学计量效应。这些不是离子强度效应，而是价态和种类特异性的。RNAP在λPR启动子处的缔合和解离速率常数都强烈依赖于盐，在NaCl中分别随[Na⁺]⁻¹²和[Na⁺]⁸变化。因此，表征该启动子处开放复合物形成的平衡常数随[Na⁺]⁻²⁰变化。在稀盐溶液中，静电相互作用和抗衡离子释放是驱动开放复合物形成的结合自由能的主要贡献因素。由于大肠杆菌（和其他细胞）的体内离子环境高度可变，这些大的盐效应几乎肯定具有生理意义。无机和有机离子（包括多胺）细胞内浓度的变化必定对转录起始以及许多其他蛋白质 - 核酸相互作用产生全局和启动子特异性的调节作用。
