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.
Highlights
The early ideas of Koppes et al (1978a,b) and Voorn et al (1993) that Escherichia coli cells grow by adding a constant length between divisions, were based on measurements of cell lengths and the rate of DNA replication in pulse-labeled cells grown in batch culture and prepared for electron microscopic autoradiography
This view has recently been revived in several studies (Amir, 2014; Campos et al, 2014; Jun and Taheri-Araghi, 2014); in the new experiments on cell size homeostasis (Campos et al, 2014; Taheri-Araghi et al, 2015; Wallden et al, 2016), the bacteria are grown in microfluidic chambers and observed by fluorescence light microscopy
The adder mechanism requires that an individual cell monitor a property that is equal in all newborn cells
Summary
The early ideas of Koppes et al (1978a,b) and Voorn et al (1993) that Escherichia coli cells grow by adding a constant length between divisions, were based on measurements of cell lengths and the rate of DNA replication in pulse-labeled cells grown in batch culture and prepared for electron microscopic autoradiography. Whereas Koppes et al (1978a,b) were able to measure only the length increment between initiation of DNA replication and the start of cell constriction, the extensive measurements of Jun and co-workers on large numbers of individual E. coli cells growing in a microfluidic “mother machine” (Wang et al, 2010) under a wide range of growth conditions covered the entire cell cycle (Taheri-Araghi et al, 2015). This could be established via the so-called transertion process that involves transcription–translation and translocation of membrane proteins (Norris, 1995; Woldringh, 2002; Rabinovitch et al, 2003) and has been proposed to interfere with the assembly of the FtsZ-ring through nucleoid occlusion (Woldringh et al, 1991; Wu and Errington, 2012)
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