Digital control of c-di-GMP in E. coli balances population-wide developmental transitions and phage sensitivity
Nucleotide-based signaling molecules (NSMs) are widespread in bacteria and eukaryotes, where they control important physiological and behavioral processes. In bacteria, NSM-based regulatory networks are highly complex, entailing large numbers of enzymes involved in the synthesis and degradation of active signaling molecules. How the converging input from multiple enzymes is transformed into robust and unambiguous cellular responses has remained unclear. Here we show that Escherichia coli converts dynamic changes of c-di-GMP into discrete binary signaling states, thereby generating heterogeneous populations with either high or low c-di-GMP. This is mediated by an ultrasensitive switch protein, PdeL, which senses the prevailing cellular concentration of the signaling molecule and couples this information to c-di-GMP degradation and transcription feedback boosting its own expression. We demonstrate that PdeL acts as a digital filter that facilitates precise developmental transitions, confers cellular memory, and generates functional heterogeneity in bacterial populations to evade phage predation. Based on our findings, we propose that bacteria apply ultrasensitive regulatory switches to convert dynamic changes of NSMs into binary signaling modes to allow robust decision-making and bet-hedging for improved overall population fitness.