Establishing how bacterial cells position the division site
In virtually all bacteria cell division is essential and tightly regulated both temporally
and spatially to ensure that cells divide precisely at the centre between segregated
chromosomes. Failure to do so can lead to cell death. The earliest event in bacterial cell
division is the polymerization of the highly conserved tubulin-like protein, FtsZ, to
form a contractile structure called the Z ring, on the inner side of the cytoplasmic
membrane at midcell and between chromosomes. The Z ring subsequently contracts
causing the cell envelope to invaginate, generating two newborn cells. Thus the Z ring
defines the position of the division site in bacterial cells.
How the Z ring is positioned precisely at midcell is a controversial topic that remains
unresolved. Division site positioning has long been believed to occur via the combined
action of two factors: the Min system and nucleoid occlusion. Both factors have been
proposed to prevent Z ring assembly along the length of the cell, allowing it to
assemble only once chromosomes segregate and nucleoid occlusion is relieved
specifically at midcell. The research described in this thesis challenges this paradigm,
providing compelling evidence that other mechanisms in addition to nucleoid occlusion
and the Min system act to position the Z ring at midcell in B. subtilis. Moreover, this
work also shows that nucleoid occlusion and the Min system do not define the Z ring
position at midcell but rather ensure that the midcell division site is utilized efficiently.
A clue to an additional mechanism for positioning the Z ring has emerged from studies
investigating the relationship between chromosome replication and Z ring position. The
nature of this relationship has remained obscure for years. Part of this thesis involves a
closer examination of this relationship. It was found that the ability to position the Z
ring at midcell is linked specifically to the progress of the initiation stage of DNA
replication, such that the frequency of Z rings at midcell increases as this stage of DNA
replication is progressively completed. Moreover, this link was found to be nucleoid
occlusion independent.
Spatial and temporal control of Z ring assembly has been widely attributed to the Min
system and nucleoid occlusion. While inactivating both systems substantially affects
cell division, it is currently unknown whether their absence affects precise midcell Z
ring positioning. This thesis deals with this question, and it was found that the
combined effect of MinCD and Noc proteins actually affects the timing and efficiency
of Z ring assembly, but not its spatial precision between nucleoids at midcell.
If Noc and MinCD proteins do not position the Z ring at midcell, what other factores
may play this role? Two hypotheses were proposed to help explain the precise Z ring
positioning observed in absence of nae and minCD: 1) Noc-independent nucleoid
occlusion or 2) factors completely independent of nucleoid occlusion position the Z
ring at midcell. Experiments designed…