Optimum Threshold Minimizes Noise in Timing of Intracellular Events

Abstract

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 l to study event timing in individual cells where cell lysis is the result of expression and accumulation of a single protein (holin) in the Escherichia coli cell membrane up to a critical threshold level. Sitedirected mutagenesis of the holin gene generated phage variants that vary in their lysis times 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 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.

An Optimal Lysis Time Maximizes Bacteriophage Fitness in Quasi-continuous Culture

“…CC-BY-NC 4.0 International license available under a was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint (which this version posted May 12, 2020. ; https://doi.org/10.1101/2020.05.11.089508 doi: bioRxiv preprint whence it nucleates generating membrane-permeabilizing holes (Wang et al 2000(Wang et al , 2003Dennehy and Wang 2011;Singh and Dennehy 2014;Kannoly et al 2020).…”

“…Lysis is the last step in the infectious cycle of lytic bacteriophage, resulting in the release of virions. Lysis timing in many phage is controlled by a single protein, holin, which accumulates in the cell membrane up to a genetically determined time, whence it nucleates generating membrane-permeabilizing holes (Wang et al 2000(Wang et al , 2003Dennehy and Wang 2011;Singh and Dennehy 2014;Kannoly et al 2020).…”

“…Lysis times are from single-cell estimates as reported in Kannoly et al, 2020. WT lysis time = 63 min.…”

“…In this study, we maintained phage λ and its host, Escherichia coli , under quasi-steady state conditions. To initiate experiments, λ holin mutants differing in their lysis times (Dennehy and Wang 2011; Singh and Dennehy 2014; Kannoly et al 2020) were added to exponentially replicating batch cultures of E. coli . After a short period of growth, λ phages were filtered from host cells, and a small fraction was transferred into a fresh exponential culture.…”

The Protein Hourglass: Exact First Passage Time Distributions for Protein Thresholds

“…It may be seen that for the Hill function there is a monotonic decrease, while for the linear case, the noise in the FPT times first decreases and then increases, yielding a U-shaped curve. It was recently shown in [39] that the CV of lysis timing for different bacteriophage mutants as a function of the effective protein threshold is also U-shaped. In that paper they attributed this effect to the effect of dilution due to bacterial growth.…”