Bistabilität bei der bakteriellen Kompetenzentwicklung
Final Report Abstract
Distinct modes of gene expression enable isogenic populations of bacteria to maintain a diversity of phenotypes and to rapidly adapt to environmental changes. Competence development for DNA transformation in Bacillus subtilis has become a paradigm for a multimodal system which implements a genetic switch through a non-linear positive feedback of a transcriptional master regulator. 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 down-regulation 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 'timewindow' for switching and is thus involved in determining the K-fraction in the bimodal population. We investigated determinants that control the kinetics of the genetic switching process from the vegetative state (B-state) to the competent state (K-state) of Bacillus subtilis, explicitly including the switching window which controls the probability for competence initiation in a cell population. For individual cells, we found that after initiation of switching the levels of the master regulator [ComK](t) increased with sigmoid shape and saturation occurred at two distinct levels of [ComK]. We analyzed the switching kinetics into the state with highest [ComK] and found saturation after a switching period of length 1.4 ± 0.3h. The duration of the switching period was robust against variations in the gene regulatory network of the master regulator, whereas the saturation levels showed large variations between individual isogenic cells. We developed a non-linear dynamics model taking into account low-number stochastic effects. The model quantitatively describes the probability and timescale of switching at the single cell level and explains why the ComK level in the K-state is highly sensitive to extrinsic parameter variations. Furthermore, the model predicts a transition from stochastic to deterministic switching at increased production rates of ComK in agreement with experimental data. Finally, we addressed the question whether the large variations of comK expression in competent cells influenced the expression level of late competence genes. We found that their expression rates were uncorrelated. We conclude that the expression of late competence proteins is induced once a threshold concentration of ComK is reached and that therefore the cell does not require tight control over the saturation level of ComK.
Publications
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(2007). Basal expression rate of comK sets a ‘switching window’ into the K-state of Bacillus subtilis. Mol. Microbiol. 63(6), 1806–1816
Leisner, M., Stingl, K., Radler, J.O. and Maier, B.
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(2008). Stochastic switching to competence. Curr. Opin. Microbiol., 11(6), 553-559
Leisner, M., Stingl, K., Frey, E., Maier, B.
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(2009). Kinetics of genetic switching into the state of bacterial competence. Biophys. J., 96(3), 1178-1188
Leisner, M., Kuhr, J.-T., Radler, J.O., Frey, E., Maier, B.
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(2010). Stochastische Differenzierung als bakterielle Überlebensstrategie. BIOSpektrum, 2, 150
Maier, B.
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Competence and transformation. (2011), In: Bacillus: Cellular and Molecular Biology, ed. Peter Graumann, Caister University Press
Maier, B.
