von Hippel P H, Revzin A, Gross C A, Wang A C
Proc Natl Acad Sci U S A. 1974 Dec;71(12):4808-12. doi: 10.1073/pnas.71.12.4808.
The regulatory system of the lactose operon has been "modeled" by a set of mass action equations and conservation constraints which describe the system at equilibrium. A "base-set" of values of binding constants and total component concentrations has been assembled from the available experimental data, and the simultaneous equations solved by computer procedures, to yield equilibrium concentrations of all the relevant molecular species. Considering the operator-repressor-inducer system alone, it is shown that the in vivo basal and induced (derepressed) levels of lac enzyme synthesis in both wild-type and certain mutant Escherichia coli can be accounted for only if binding of repressor and repressor-inducer complexes to non-specific DNA sites is included in the calculations as an integral component of the ovrall control system. A similar approach was applied to the RNA polymerase-promoter system to show that sigma factor may modulate the general level of transcription in the cell by "inducing" polymerase off non-specific DNA binding sites, thus making it available to promoters. Competitive and non-competitive models for the interaction of repressor and polymerase at the lac operon can, in principle, be distinguished by these computational procedures, though data sufficient to permit unambiguous differentiation between the models are not available at this time. However, for any competitive binding model the results show that repression in the entire (operator-repressor-RNA polymerase-lac promoter) system can occur only because non-specific binding of the regulatory proteins reduces the concentration of free polymerase, relative to that of repressor, to appropriate levels.
乳糖操纵子的调控系统已通过一组质量作用方程和守恒约束进行了“建模”，这些方程和约束描述了处于平衡状态的系统。已根据现有实验数据汇总了一组结合常数和总组分浓度的“基础集”，并通过计算机程序求解联立方程，以得出所有相关分子物种的平衡浓度。仅考虑操纵子 - 阻遏物 - 诱导物系统，结果表明，只有在计算中纳入阻遏物和阻遏物 - 诱导物复合物与非特异性DNA位点的结合作为整体控制系统的一个组成部分时，才能解释野生型和某些突变型大肠杆菌中乳糖酶合成的体内基础水平和诱导（去阻遏）水平。类似的方法应用于RNA聚合酶 - 启动子系统，结果表明，σ因子可能通过“诱导”聚合酶脱离非特异性DNA结合位点来调节细胞中的转录总体水平，从而使其能够与启动子结合。原则上，阻遏物与聚合酶在乳糖操纵子处相互作用的竞争和非竞争模型可以通过这些计算程序来区分，尽管目前尚无足够的数据来明确区分这些模型。然而，对于任何竞争结合模型，结果表明，整个（操纵子 -阻遏物 - RNA聚合酶 - 乳糖启动子）系统中的阻遏作用之所以会发生，仅仅是因为调节蛋白的非特异性结合将游离聚合酶的浓度相对于阻遏物的浓度降低到了适当水平。
