Everything under one roof

Distributing gene copies reliably to the two daughter cells during cell division is a crucial process of life. Unfortunately, however, little is known about its mechanisms, especially in bacteria, which make up the majority of organisms on earth. In addition to chromosomes, bacteria possess other genetic elements: small, ring-shaped, self-replicating plasmids that give them additional capabilities. Due to their small size and ease of production, plasmids are an important tool in basic research as well as in biotechnological applications. At the same time, they pose a problem for public health from a human perspective, as some of them are responsible for virulence factors or antibiotic resistance.

For plasmids and chromosomes with a low copy number, random distribution during cell division is not reliable enough: an active mechanism is needed so that both daughter cells inherit at least one plasmid. In most cases, this is mediated by a three-component system called Par. The most common of these, ParABS, consists of two proteins and a DNA binding sequence. However, it is still unknown how this ensures stable and precise inheritance. In particular, it remains unclear whether the plasmids are directed to specific target positions or whether oscillating movements occur.

Dynamics of cell cycles in living cells

Now the Max Planck research team led by Seán Murray has been able to uncover the true nature of the ParABS system. In an interdisciplinary approach, the researchers tracked the dynamics of many thousands of cell cycles in living cells and combined their data with a mathematical model. They analysed two different, distantly related systems from the human intestinal bacterium Escherichia coli:  the F plasmid, the main carrier of antibiotic resistance genes, and pB171, the virulence plasmid of a pathogenic strain.

Intriguingly, their results show that both systems act in the same way at a fundamental level. "Our model reveals that there is a dominant mode of plasmid inheritance: directed positioning, in which the plasmids are actively moved to specific subcellular positions," says the first author of the study, Robin Köhler.According to the model, the ParABS system positions the plasmids at regular intervals across the cell, operating just below the threshold of oscillatory instability. He adds: "Our simulations show that this reduces the energy consumption of the system to a minimum."

This study was made possible by the interdisciplinary combination of high-throughput experiments and mathematical modelling. According to research group leader Seán Murray, mathematical models built on a solid experimental foundation can be very successful in finding the underlying principle of a biological mechanism: „Our model unifies previously contradictory models and it provides a robust mechanistic basis for self-organised distribution of plasmids during cell division. Furthermore, it can potentially be applied to other Par systems. In particular, chromosomal ParABS systems, which still present us with many questions, could benefit greatly from our work.

Source: Press release Max Planck Society, 15 November 2022