Dynamics of Cell Fate Decision Making Between Sporulation and Competence in Bacillus Subtilis
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During multipotent differentiation cells must reliably make a cell fate decision under a variety of conditions, yet remain sensitive to changes in extracellular environment. It is unclear how the cells reconcile these seemingly contradictory requirements. To complicate the issue, the cells often face a decision between multiple fates mediated by the respective differentiation programs which could become active at once. How cells make a specific cell fate choice when presented with several possibilities is a fundamental, yet poorly resolved question. To study cell-fate decision-making dynamics, I utilized the soil bacterium Bacillus subtilis which under stress can either become competent for DNA uptake or undergo sporulation. The master regulator of sporulation is the transcription factor Spo0A. Single cell measurements of Spo0A dynamics along with activities of stage-specific sporulation reporters Spo0F, SpoIIE and SpoIIR revealed the reversible and noisy progression of sporulation up until the final irreversible decision point. Mathematical modeling suggested that such strategy might be advantageous for coping with unpredictable environment. The alternative cell fate of competence is controlled by the transcription factor ComK. Using time-lapse fluorescence microscopy, I quantitatively measured the activities of Spo0A and ComK, along with other cross-regulatory genes, simultaneously in single B. subtilis cells. I found that, surprisingly, sporulation and competence progressed independently in the same cell without cross-regulation up to the final decision point. This finding was confirmed by the discovery of cells in a conflicted state that progressed to sporulation despite the expression of ComK. Measurements of gene expression dynamics in these cells revealed key differences in the relative timing of differentiation programs. To investigate the importance of relative timing, I altered it by engineering artificial cross-regulatory links between the sporulation and competence genetic circuits. Results favor a simple model for cellular decision-making that does not require intricate cross-regulation prior to the decision. Rather, cell fate choice appears to be the outcome of a "molecular race" between independently progressing differentiation programs. This temporal competition mechanism provides a simple, yet efficient way to generate mutually exclusive cell fates. Investigation of the benefits and limitations of such strategy opens a promising venue for future studies.