Phenotypic switching of Escherichia coli cells containing cyclic digenic systems with negative feedback upon changes in cultivation conditions
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One of the mechanisms for the epigenetic control of cell phenotypes is based on switching the functioning regimes of bistable gene networks, which can maintain the two alternative levels of gene expression under the same conditions. Cyclic digenic systems with negative feedback represent an example of a simple bistable gene network. Cells carrying artificial cyclic digenic systems on plasmids inherit each alternative phenotype upon exponential growth on rich medium during several cell generations. The action of specific inducers is necessary for switching. In this work, the impact of changes in cell cultivation conditions on the phenotypic composition of the clonal Escherichia coli cell population containing artificial cyclic digenic systems with negative feedback was studied. Phenotypes differ with respect to the expression level of marker proteins: β-galactosidase and GFP. Slow growth on a medium containing little-available carbon sources was shown to cause the transition from the phenotype Lac− to Lac+ in the absence of inducers. Phenotypic switching cannot be explained by transcriptional activation of the lactose operon, because 80 ± 15% of cells inherit the acquired phenotype after replating bacteria on rich medium. Inheritance of the phenotype Lac− in batch culture depends on the medium and duration of cultivation. Dynamics of changes in the activity of β-galactosidase and culture fluorescence suggests that a decrease in the level of metabolism resulted in the switch of these cyclic systems from bistable to monostable functioning regime, which corresponds to the Lac+ phenotype with respect to the ratio of regulatory proteins. Thus, the instability of growth conditions may cause phenotypic heterogeneity in the clonal population of cells containing bistable gene networks.
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