The Origin of Cell Fate Heterogeneity in Bacillus Subtilis

Abstract

A central question in biological development is how individual cells in a common environment are able to adopt distinct cell fates. In response to nutrient deprivation, Bacillus subtilis cells initiate the formation of a dormant cell type, the spore, in a process requiring the activation of many genes and operons. Entry into sporulation is governed by the phosphorylation of a key regulator (Spo0A), which is mediated by successive phosphotransfers from the kinase, KinA, to two phosphorelay proteins, which in turn hand off the phosphoryl group to Spo0A. Spo0A~P, in turn, sets in motion a chain of events that leads to the asymmetric activation of the transcription factor σF, a hallmark for entry into sporulation. I used a microfluidic-based platform in conjunction with fluorescent reporters for promoters that require low or high levels of Spo0A~P and a promoter controlled by σF to investigate the role of intrinsic noise at the level of individual cells in the decision to enter sporulation. The principal conclusions and observations from this analysis were: (1) that entry into sporulation (activation of σF) is heterogeneous in time and occurs with approximately a constant probability after adaptation to sporulation-inducing conditions; (2) activation of σF is predicted by Spo0A~P reaching a high, threshold level, which occurs heterogeneously over time; (3) the source of the noise is not at the level of the synthesis, accumulation or activity of KinA; and (4) noise seems to originate from the phosphorelay itself.