Optimum Threshold Minimizes Noise in Timing of Intracellular Events

How the noisy expression of regulatory proteins affects timing of intracellular events is an intriguing fundamental problem that influences diverse cellular processes. Here we use the bacteriophage {lambda} to study event timing in individual cells where cell lysis is the result of expression and accumulation of a single protein (holin) in the Escherchia coli cell membrane up to a critical threshold level. Site-directed mutagenesis of the holin gene was used to generate phage variants that vary in their timing of lysis from 30 to 190 min. Observation of the lysis times of single cells reveals an intriguing finding - the noise in lysis timing first decreases with increasing lysis time to reach a minimum, and then sharply increases at longer longer lysis times. A mathematical model with stochastic expression of holin together with dilution from cell growth was sufficient to explain the non-monotonic noise profile, and identify holin accumulation thresholds that generate precision in lysis timing.