An analog to digital converter controls bistable transfer competence development of a widespread bacterial integrative and conjugative element

Conjugative transfer of the integrative and conjugative element ICE clc in Pseudomonas requires development of a transfer competence state in stationary phase, which arises only in 3-5% of individual cells. The mechanisms controlling this bistable switch between non-active and transfer competent cells have long remained enigmatic. Using a variety of genetic tools and epistasis experiments in P. putida , we uncovered an ‘upstream’ cascade of three consecutive transcription factor-nodes, which controls transfer competence initiation. Initiation activates a feedback loop, which stochastic modeling and experimental data demonstrated acts as a scalable converter of unimodal input to bistable output. The feedback loop further enables prolonged production of a transcription factor that ensures ‘downstream’ transfer competence formation in activated cells. Phylogenetic analyses showed that the ICE clc regulatory factors are widespread among Gamma- and Beta -proteobacteria, highlighting its evolutionary conservation and prime importance to control the behaviour of this wide family of conjugative elements.