In cells, specific regulators often compete for limited amounts of a core enzymatic resource. It is typically assumed that competition leads to partitioning of core enzyme molecules among regulators at constant levels. Alternatively, however, different regulatory species could time share, or take turns utilizing, the core resource. Using quantitative time-lapse microscopy, we analyzed sigma factor activity dynamics, and their competition for RNA polymerase, in individual Bacillus subtilis cells under energy stress. Multiple alternative sigma factors were activated in ∼1-hr pulses in stochastic and repetitive fashion. Pairwise analysis revealed that two sigma factors rarely pulse simultaneously and that some pairs are anti-correlated, indicating that RNAP utilization alternates among different sigma factors. Mathematical modeling revealed how stochastic time-sharing dynamics can emerge from pulse-generating sigma factor regulatory circuits actively competing for RNAP. Time sharing provides a mechanism for cells to dynamically control the distribution of cell states within a population. Since core molecular components are limiting in many other systems, time sharing may represent a general mode of regulation.

This work was supported by NIH grants R01 GM079771B and R01 HD075605A (to M.B.E.), T32 GM07616 (to J.P.), and NIHGMSK99BM118910 (to S.H.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH. It was also supported by National Science Foundation grant 1547056 and the Institute for Collaborative Biotechnologies through grant W911NF-09-0001 (KK9150) from the U.S. Army Research Office. The content of the information does not necessarily reflect the position or the policy of the government, and no official endorsement should be inferred. J.G.-O. and M.D. were supported by the Spanish Ministry of Economy and Competitiveness (MINECO) and FEDER (project FIS2015-66503-C3-1-P), the ICREA Academia program, and the Maria de Maeztu Program for Units of Excellence in R&D (MINECO, project MDM-2014-0370). Work in the Locke laboratory was supported by the European Research Council under the European Union's Seventh Framework Program (FP/2007-2013)/ERC Grant Agreement 338060, a fellowship from the Gatsby Foundation (GAT3272/GLC), and an award from the Human Frontier Science Program (CDA00068/2012).