Disruption of glycogen metabolism alters cell size inEscherichia coli

Abstract

The availability of nutrients impacts cell size and growth rate in many organisms. Research in E. coli has traditionally focused on the influence of exogenous nutrient sources on cell size through their effect on growth and cell cycle progression. Utilising a set of mutants where three genes involved in glycogen degradation -glycogen phosphorylase (glgP), glycogen debranching enzyme (glgX) and maltodextrin phosphorylase (malP) -were disrupted, we examined if endogenous polyglucan degradation affects cell size. It was found that mutations to malP increased cell lengths and resulted in substantial heterogeneity of cell size. This was most apparent during exponential growth and the phenotype was unaccompanied by alterations in Z-ring occurrence, cellular FtsZ levels and generation times. ∆malP mutant cells did, however, accumulate increased DnaA amounts at late growth stages indicating a potential effect on DNA replication. Replication run-out experiments demonstrated that this was indeed the case, and that DNA replication was also affected in the other mutants. Bacteria with a disruption in glgX accumulated glycogen and protein inclusion bodies that coincided with each other at inter-nucleoid and polar regions.Robust adaptive survival strategies are employed by bacteria to promote fitness. Most notably, when nutrients are abundant, bacterial cells demonstrate increases in length, width and growth rate 1 which permit parental cells to promote vigour of future generations by giving rise to larger daughter cells 2 . Enhanced rates of macromolecular biosynthesis accompany nutrientdependent size increases to ensure that larger cells are born with more DNA, RNA and protein 3-6 . This positive scaling relationship between cell size and external nutrient-imposed growth rate has historically been termed Schaechter's nutrient growth law 1 . Escherichia coli is known to passively correct deviant changes to cell size by adding a constant volume of cellular material (cell unit) between division events, regardless of size at birth 7,8 which has led to the proposal of what is known as the adder model 9 .Under steady-state conditions, the cell unit postulated in the adder model is dictated by two mechanisms 10 . The first is an initiation adder that ensures DNA origin firing is triggered only when the cell has grown and accumulated protein factors, such as DnaA, to a threshold required to initiate DNA replication. Synchronous origin firing under conditions of slow growth results in the birth of two daughters, each containing one parental chromosome; however, in environments that promote rapid growth, cell division can occur at a rate faster than the time it takes to duplicate the genome. Bacteria overcome this problem by initiating overlapping replication cycles, so daughter cells are born with two, four or even eight replication origins that ensure genomic integrity is maintained when the cell is faced with rapid division rates 11 . Several studies have reported that the volume of the cell unit proposed in the adder model is prop...