Robust replication initiation from coupled homeostatic mechanisms

TL;DR

This paper presents a mathematical model showing that the interaction of titration and activation mechanisms of DnaA protein controls robust replication initiation in E. coli across different growth rates, explaining precise cell cycle regulation.

Contribution

The study introduces a combined model of titration and activation mechanisms for DnaA, explaining how they jointly ensure stable replication cycles in E. coli.

Findings

Titration alone stabilizes cycles at low growth rates but causes reinitiation at high rates.

DnaA activation switch is crucial for stability at high growth rates.

Both mechanisms together ensure robust replication across all growth conditions.

Abstract

The bacterium Escherichia coli initiates replication once per cell cycle at a precise volume per origin and adds an on average constant volume between successive initiation events, independent of the initiation size. Yet, a molecular model that can explain these observations has been lacking. Experiments indicate that E. coli controls replication initiation via titration and activation of the initiator protein DnaA. Here, we study by mathematical modelling how these two mechanisms interact to generate robust replication-initiation cycles. We first show that a mechanism solely based on titration generates stable replication cycles at low growth rates, but inevitably causes premature reinitiation events at higher growth rates. In this regime, the DnaA activation switch becomes essential for stable replication initiation. Conversely, while the activation switch alone yields robust rhythms at high growth rates, titration can strongly enhance the stability of the switch at low growth rates. Our analysis thus predicts that both mechanisms together drive robust replication cycles at all growth rates. In addition, it reveals how an origin-density sensor yields adder correlations.