Noise in a phosphorelay drives stochastic entry into sporulation in Bacillus subtilis

Entry into sporulation in Bacillus subtilis is governed by a phosphorelay in which phosphoryl groups from a histidine kinase are successively transferred via relay proteins to the response regulator Spo0A. Spo0A similar to P, in turn, sets in motion events that lead to asymmetric division and activation of the cell-specific transcription factor sigma(F), a hallmark for entry into sporulation. Here, we have used a microfluidics-based platform to investigate the activation of Spo0A and sigma(F) in individual cells held under constant, sporulation-inducing conditions. The principal conclusions were that: (i) activation of sigma(F) occurs with an approximately constant probability after adaptation to conditions of nutrient limitation; (ii) activation of sigma(F) is tightly correlated with, and preceded by, Spo0A similar to P reaching a high threshold level; (iii) activation of Spo0A takes place abruptly just prior to asymmetric division; and (iv) the primary source of noise in the activation of Spo0A is the phosphorelay. We propose that cells exhibit a constant probability of attaining a high threshold level of Spo0A similar to P due to fluctuations in the flux of phosphoryl groups through the phosphorelay.