The bacterial cell cycle is traditionally divided into three stages: the period between division (cell ‘birth’) and the initiation of chromosome replication (known as the B period); the period required for replication (known as the C period); and the time between the end of replication and completion of division (known as the D period) (). In the enteric organism Escherichia coli
and the spore former Bacillus subtilis
, DNA replication begins at a single origin (oriC
) on a single circular chromosome. Replication proceeds bidirectionally around the circumference of the chromosome, terminating at a region opposite oriC
. During replication the chromosome remains in a condensed, highly ordered structure that is known as the nucleoid (see REF. 3
for a review of chromosome replication). Division is initiated near the end of chromosome segregation by the formation of a cytokinetic ring at the nascent division site. The tubulin-like protein FtsZ serves as the foundation for assembly of this ring and is required for recruitment of the division machinery (see REF. 4
for a review of bacterial cell division). Nutrient availability and growth rate could potentially affect any of the above steps.
Our understanding of the bacterial cell cycle under different growth conditions derives largely from early physiological studies of B. subtilis
and E. coli5
. These studies indicated that, at constant temperature, mass doubling time decreases in response to increases in nutrient availability; however, both the C period and the D period remain essentially constant. Consequently, under nutrient-rich conditions, both E. coli
and B. subtilis
reach growth rates at which the period required for chromosome replication and segregation is greater than the mass doubling time. To resolve this paradox, rapidly growing cells initiate new rounds of chromosome replication before completing the previous round, a situation that results in two, four or even eight rounds of replication proceeding simultaneously. This phenomenon, which was first discovered in B. subtilis
and termed ‘multifork replication’ (REF. 6
), was formalized and further investigated by Cooper and Helmstetter in their influential 1968 paper5
). Notably, Cooper and Helmstetter’s work illuminated how cells balance largely constant rates of replication fork progression with nutrient-dependent changes in mass doubling time, by initiating replication and dividing more frequently when growing faster.
Box 1. Cooper and Helmstetter’s model
Replication during slow growth
In slow-growing bacterial cells (with a mass doubling time >C + D period), there is a single round of replication per division cycle. This type of growth resembles that of eukaryotes in that there is a gap (the B period), a period in which DNA replication takes place (the C period) and finally a period of chromosome segregation and cell division (the D period). During replication each cell has only two copies of the origin region (oriC) and one copy of the terminus (terC) (see the figure, part a).
Replication during fast growth (multifork replication)
In rapidly growing cells (with a mass doubling time ≤C + D period), each chromosome re-initiates a new round of replication before the first round has terminated, although only one round is initiated per cell division. Multifork replication ensures that at least one round of replication is finished before cytokinesis, to guarantee that each daughter cell receives at least one complete genome. During multifork replication cells can have four or more copies of the region proximal to oriC and one copy of the region proximal to terC (see the figure, part b). This imbalance has implications for gene expression levels as well as for the activity of the initiator protein DnaA.
Although arguably one of the most important insights in the field in the past 40 years, Cooper and Helmstetter’s model is limited in that it views the cell cycle as a single process, in which replication initiation is the triggering event that determines the timing of all subsequent steps in replication and cell division. This view does not take into consideration the effects of nutrients and metabolic status on events that occur after replication initiation, nor does it explain how cell cycle events are coordinated with mass doubling to ensure that new rounds of replication are initiated only once per division cycle and cell size homeostasis is maintained. Recent work suggests that, instead of being a single process, the bacterial cell cycle is a set of coordinated but independent events (see REFS 7,8
for an eloquent presentation of this model). This more nuanced view is the model to which we subscribe.
Multifork replication is not a universal feature of the bacterial life cycle: the aquatic bacterium Caulobacter crescentus
has temporally compartmentalized cell cycle stages, a situation analogous to the eukaryotic cell cycle9
. For simplicity, however, in this Review we treat B. subtilis
and E. coli
as representative Gram-positive and Gram-negative bacteria, respectively. In addition, we use the term ‘division cycle’ instead of cell cycle to refer to the period of time between the birth of a cell and its own subsequent division. This Review focuses first on chromosome replication and then on cell division, but these should not be regarded as independent processes.