Zander Dominique, Samaga Daniel, Straube Ronny, Bettenbrock Katja
Max-Planck-Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany.
Max-Planck-Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany.
Biophys J. 2017 May 9;112(9):1984-1996. doi: 10.1016/j.bpj.2017.03.038.
The Escherichia coli lac operon is regulated by a positive feedback loop whose potential to generate an all-or-none response in single cells has been a paradigm for bistable gene expression. However, so far bistable lac induction has only been observed using gratuitous inducers, raising the question about the biological relevance of bistable lac induction in the natural setting with lactose as the inducer. In fact, the existing experimental evidence points to a graded rather than an all-or-none response in the natural lactose uptake system. In contrast, predictions based on computational models of the lactose uptake pathway remain controversial. Although some argue in favor of bistability, others argue against it. Here, we reinvestigate lac operon expression in single cells using a combined experimental/modeling approach. To this end, we parameterize a well-supported mathematical model using transient measurements of LacZ activity upon induction with different amounts of lactose. The resulting model predicts a monostable induction curve for the wild-type system, but indicates that overexpression of the LacI repressor would drive the system into the bistable regime. Both predictions were confirmed experimentally supporting the view that the wild-type lac induction circuit generates a graded response rather than bistability. More interestingly, we find that the lac induction curve exhibits a pronounced maximum at intermediate lactose concentrations. Supported by our data, a model-based analysis suggests that the nonmonotonic response results from saturation of the LacI repressor at low inducer concentrations and dilution of Lac enzymes due to an increased growth rate beyond the saturation point. We speculate that the observed maximum in the lac expression level helps to save cellular resources by limiting Lac enzyme expression at high inducer concentrations.
大肠杆菌乳糖操纵子受正反馈回路调控，其在单细胞中产生全或无反应的潜力一直是双稳态基因表达的范例。然而，到目前为止，仅在使用 gratuitous 诱导剂时观察到双稳态乳糖诱导，这就引发了关于在以乳糖为诱导剂的自然环境中双稳态乳糖诱导的生物学相关性的问题。事实上，现有的实验证据表明，在天然乳糖摄取系统中存在的是分级反应而非全或无反应。相比之下，基于乳糖摄取途径计算模型的预测仍存在争议。尽管一些人支持双稳态，但另一些人则反对。在这里，我们使用实验/建模相结合的方法重新研究单细胞中的乳糖操纵子表达。为此，我们通过用不同量的乳糖诱导后对 LacZ 活性进行瞬态测量，对一个得到充分支持的数学模型进行参数化。所得模型预测野生型系统的诱导曲线是单稳态的，但表明 LacI 阻遏蛋白的过表达会使系统进入双稳态状态。这两个预测都通过实验得到了证实，支持了野生型乳糖诱导回路产生分级反应而非双稳态的观点。更有趣的是，我们发现乳糖诱导曲线在中等乳糖浓度处呈现出明显的最大值。在我们数据的支持下，基于模型的分析表明，这种非单调反应是由于低诱导剂浓度下 LacI 阻遏蛋白的饱和以及超过饱和点后生长速率增加导致 Lac 酶稀释所致。我们推测，观察到的乳糖表达水平最大值有助于通过在高诱导剂浓度下限制 Lac 酶表达来节省细胞资源。
