Does the eclipse limit bacterial nucleoid complexity and cell width?

Abstract

Cell size of bacteria M is related to 3 temporal parameters: chromosome replication time C, period from replication-termination to subsequent division D, and doubling time τ. Steady-state, bacillary cells grow exponentially by extending length L only, but their constant width W is larger at shorter τ's or longer C's, in proportion to the number of chromosome replication positions n (= C/τ), at least in Escherichia coli and Salmonella typhimurium. Extending C by thymine limitation of fast-growing thyA mutants result in continuous increase of M, associated with rising W, up to a limit before branching. A set of such puzzling observations is qualitatively consistent with the view that the actual cell mass (or volume) at the time of replication-initiation Mi (or Vi), usually relatively constant in growth at varying τ's, rises with time under thymine limitation of fast-growing, thymine-requiring E. coli strains. The hypothesis will be tested that presumes existence of a minimal distance l_min between successive moving replisomes, translated into the time needed for a replisome to reach l_min before a new replication-initiation at oriC is allowed, termed Eclipse E. Preliminary analysis of currently available data is inconsistent with a constant E under all conditions, hence other explanations and ways to test them are proposed in an attempt to elucidate these and other results. The complex hypothesis takes into account much of what is currently known about Bacterial Physiology: the relationships between cell dimensions, growth and cycle parameters, particularly nucleoid structure, replication and position, and the mode of peptidoglycan biosynthesis. Further experiments are mentioned that are necessary to test the discussed ideas and hypotheses.

Keywords: Bacterial cell cycle; Eclipse; Growth; Nucleoid complexity; Replication and division; Size and dimensions.

Fig. 1

Electron micrograph of a mixture of two E. coli B/r cultures on agar filters. The big cells were grown with a doubling time τ = 22 min; the small cells, with τ = 150 min. Adapted from Zaritsky & Woldringh . Arrows indicate the transition enigma.

Fig. 2

The classic nutritional shift-up experiment (Adapted from Ref. .). The red oval depicts the maintenance period (∼65 min) of cell division rate.

Fig. 3

Dimensional Rearrangement during nutritional shift-up (Adapted from Ref. .). The red oval depicts a temporary enhanced rate of cell division upon the upshift.

Fig. 4

Electron micrographs of cells 60 min after a nutritional shift-up from τ₁ = 72’ → τ₂ = 24’. The nucleoids appear as electron-transparent regions. Red arrows indicate constriction sites, blue arrows—tapered tips. From Ref. .

Fig. 5

Rate of increase in average cell mass with time (Adapted from Ref. .). E. coli strain P178 was grown in glucose M9 containing the following [T] (in μg ml⁻¹): (a) 0.4, (b) 0.5, (c) 1, (d) 2, (e) 5, (f) 30.

Fig. 6

Schematic overview of 4 physiological states of E. coli cells with different sizes, shapes and chromosome configurations. The initial state represents a cell with (τ, C, D) = (40, 40, 20) min. The drawings are only roughly to scale. Note: nucleoid complexity (NC) only changes in states (a) and (d), from a chromosome with 4 to one with 8 origins at the end of the cell cycle. For exact chromosome configurations, see Norbert Vischer's CCSim program , ; http://sils.fnwi.uva.nl/bcb/cellcycle/).