Dekel Erez, Alon Uri
Department of Molecular Cell Biology and Department of Physics of Complex Systems, The Weizmann Institute of Science, Rehovot 76100, Israel.
Nature. 2005 Jul 28;436(7050):588-92. doi: 10.1038/nature03842.
Different proteins have different expression levels. It is unclear to what extent these expression levels are optimized to their environment. Evolutionary theories suggest that protein expression levels maximize fitness, but the fitness as a function of protein level has seldom been directly measured. To address this, we studied the lac system of Escherichia coli, which allows the cell to use the sugar lactose for growth. We experimentally measured the growth burden due to production and maintenance of the Lac proteins (cost), as well as the growth advantage (benefit) conferred by the Lac proteins when lactose is present. The fitness function, given by the difference between the benefit and the cost, predicts that for each lactose environment there exists an optimal Lac expression level that maximizes growth rate. We then performed serial dilution evolution experiments at different lactose concentrations. In a few hundred generations, cells evolved to reach the predicted optimal expression levels. Thus, protein expression from the lac operon seems to be a solution of a cost-benefit optimization problem, and can be rapidly tuned by evolution to function optimally in new environments.
不同的蛋白质具有不同的表达水平。目前尚不清楚这些表达水平在多大程度上与其所处环境相适应。进化理论表明，蛋白质表达水平能使适应性最大化，但作为蛋白质水平函数的适应性很少被直接测量。为了解决这个问题，我们研究了大肠杆菌的乳糖操纵子系统，该系统使细胞能够利用乳糖作为碳源进行生长。我们通过实验测量了由于乳糖蛋白的产生和维持所带来的生长负担（成本），以及当乳糖存在时乳糖蛋白所带来的生长优势（益处）。由益处与成本之差给出的适应性函数预测，对于每种乳糖环境，都存在一个使生长速率最大化的最佳乳糖表达水平。然后我们在不同乳糖浓度下进行了系列稀释进化实验。在几百代内，细胞进化到达到预测的最佳表达水平。因此，来自乳糖操纵子的蛋白质表达似乎是一个成本效益优化问题的解决方案，并且可以通过进化在新环境中快速调整以实现最佳功能。
