Leisner Madeleine, Stingl Kerstin, Rädler Joachim O, Maier Berenike
Institut für Allgemeine Zoologie und Genetik, Westfälische Wilhelms Universität, Schlossplatz 5, 48149 Münster, Germany.
Mol Microbiol. 2007 Mar;63(6):1806-16. doi: 10.1111/j.1365-2958.2007.05628.x.
Bacillus subtilis cell population divides into a competent fraction and a non-competent fraction in the stationary phase. The transition from the non-competent state (with basal ComK concentration) to the K-state (with high ComK concentration) behaves like a bistable switch. To better understand the mechanism that sets the fraction of cells that switch into the K-state (K-fraction), we characterized the basal comK expression in individual non-competent cells and found a large cell-to-cell variation. Basal expression rate increased exponentially, reached a maximum and decreased towards zero in the stationary phase. Concomitantly, the intrinsic switching rate increased and decreased with a time lag. When switching was induced prematurely by reduction of ComK proteolysis, the K-fraction increased strongly. Our data support a model in which the average basal level of ComK raises during late exponential phase and due to noise in basal comK expression only those cells that are on the high end of comK expression trigger the autocatalytic feedback for ComK transcription. We show that a subsequent shut-down of basal expression rate sets a 'time-window' for switching and is thus involved in determining the K-fraction in the bimodal population.
枯草芽孢杆菌细胞群体在稳定期分为感受态部分和非感受态部分。从非感受态状态（ComK浓度处于基础水平）到K状态（ComK浓度高）的转变表现为双稳态开关。为了更好地理解决定转变为K状态的细胞比例（K比例）的机制，我们对单个非感受态细胞中的基础comK表达进行了表征，发现细胞间存在很大差异。基础表达率呈指数增加，达到最大值，然后在稳定期降至零。与此同时，内在转换率随时间滞后而增加和减少。当通过减少ComK蛋白水解过早诱导转换时，K比例大幅增加。我们的数据支持这样一个模型：在指数生长后期，ComK的平均基础水平升高，并且由于基础comK表达中的噪声，只有那些comK表达处于高端的细胞触发ComK转录的自催化反馈。我们表明，随后基础表达率的关闭为转换设定了一个“时间窗口”，因此参与决定双峰群体中的K比例。
