Different Amounts of DNA in Newborn Cells of Escherichia coli Preclude a Role for the Chromosome in Size Control According to the “Adder” Model

Abstract

According to the recently-revived adder model for cell size control, newborn cells of Escherichia coli will grow and divide after having added a constant size or length, ΔL, irrespective of their size at birth. Assuming exponential elongation, this implies that large newborns will divide earlier than small ones. The molecular basis for the constant size increment is still unknown. As DNA replication and cell growth are coordinated, the constant ΔL could be based on duplication of an equal amount of DNA, ΔG, present in newborn cells. To test this idea, we measured amounts of DNA and lengths of nucleoids in DAPI-stained cells growing in batch culture at slow and fast rates. Deeply-constricted cells were divided in two subpopulations of longer and shorter lengths than average; these were considered to represent large and small prospective daughter cells, respectively. While at slow growth, large and small prospective daughter cells contained similar amounts of DNA, fast growing cells with multiforked replicating chromosomes, showed a significantly higher amount of DNA (20%) in the larger cells. This observation precludes the hypothesis that ΔL is based on the synthesis of a constant ΔG. Growth curves were constructed for siblings generated by asymmetric division and growing according to the adder model. Under the assumption that all cells at the same growth rate exhibit the same time between initiation of DNA replication and cell division (i.e., constant C+D-period), the constructions predict that initiation occurs at different sizes (Li) and that, at fast growth, large newborn cells transiently contain more DNA than small newborns, in accordance with the observations. Because the state of segregation, measured as the distance between separated nucleoids, was found to be more advanced in larger deeply-constricted cells, we propose that in larger newborns nucleoid separation occurs faster and at a shorter length, allowing them to divide earlier. We propose a composite model in which both differential initiation and segregation leads to an adder-like behavior of large and small newborn cells.

A bacterial size law revealed by a coarse-grained model of cell physiology

“…Yet it indicates that cell division is not enslaved to the process controlling DNA replication initiation. Also, recent experiments reported different amounts of DNA in newborn cells, suggesting the chromosome is not involved in size homeostasis or responsible for the adder behavior [59]. Here we find that homeostatic properties at the level of cell division appear consistent with the structural model of division control.…”

Bacterial Growth Control Mechanisms Inferred from Multivariate Statistical Analysis of Single-Cell Measurements

“…Several new investigations that have been reported recently support this possibility. [21][22][23] However, examining it experimentally is beyond the scope of this study, and a full investigation of the role of the DNA content is the subject of ongoing research efforts by our group.…”

A bacterial size law revealed by a coarse-grained model of cell physiology

“…Yet it indicates that cell division is not enslaved to the process controlling DNA replication initiation. Also, recent experiments reported different amounts of DNA in newborn cells, suggesting the chromosome is not involved in size homeostasis or responsible for the adder behavior (Huls et al 2018). Here we find that homeostatic properties at the level of cell division appear consistent with the structural model of division control.…”