Prokaryotic Development: A New
Player on the Cell Cycle Circuit
Craig Stephens
A genetic regulatory circuit recently described in the
bacterium Caulobacter crescentus generates
reciprocal oscillations in the abundance of two key
transcription factors to control landmark events in
the cell cycle.
Proliferating cells progress through the cell cycle in
orderly fashion, carefully regulating the production,
activity and placement of proteins involved in critical
processes such as chromosome replication and
segregation, organelle development and cell division.
Decades of work on the eukaryotic cell cycle,
motivated by obvious implications for understanding
both cancer and embryonic development, has yielded
profound insights into its basic regulatory circuitry [1].
Our understanding of the prokaryotic cell cycle has
lagged far behind, but work in model systems such as
the dimorphic bacterium Caulobacter crescentus is
unveiling at least some of the regulatory strategies
employed by prokaryotes to guide this essential
process. Most recently, Holtzendorff et al. [2] report
that levels of two Caulobacter transcription factors
oscillate in reciprocal fashion over the course of the
cell cycle, comprising a regulatory circuit directing
landmark events of the cell cycle.
Caulobacter are aquatic bacteria that progress
through an invariant series of developmental events
each cell cycle [3] (Figure 1A). The ‘swarmer’ cell, a
motile stage equipped to seek out nutrients, is
analogous to the G1 stage of the eukaryotic cell cycle
in that chromosome replication is blocked (Figure 1A).
The eventual transition into S phase — chromosome
replication — is marked by the swarmer cell shedding
its polar flagellum and replacing it with an adhesive
stalk. As the sessile stalked cell elongates, a new
flagellum is synthesized at the unoccupied cell pole.
This event depends on ongoing DNA replication,
through a poorly understood checkpoint mechanism
[4]. Subsequent cell division also depends on completion of DNA replication and proper segregation of the
daughter chromosomes [4].
Until the work of Holtzendorff et al. [2], the CtrA transcription factor was the only known ‘master regulator’
of the Caulobacter cell cycle. CtrA controls initiation of
DNA replication through binding to the origin [5], and
directly controls the expression of at least 95 cell-cycleregulated genes involved in diverse processes, including cell division, flagellar motility, adhesion and DNA
methylation [6]. CtrA is subject to multiple forms of regulation. Expression of ctrA is cell-cycle regulated, and
the activity of CtrA depends on its phosphorylation by
Prokaryotic Development: A NewPlayer on the Cell Cycle CircuitCraig StephensA genetic regulatory circuit recently described in thebacterium Caulobacter crescentus generatesreciprocal oscillations in the abundance of two keytranscription factors to control landmark events inthe cell cycle.Proliferating cells progress through the cell cycle inorderly fashion, carefully regulating the production,activity and placement of proteins involved in criticalprocesses such as chromosome replication andsegregation, organelle development and cell division.Decades of work on the eukaryotic cell cycle,motivated by obvious implications for understandingboth cancer and embryonic development, has yieldedprofound insights into its basic regulatory circuitry [1].Our understanding of the prokaryotic cell cycle haslagged far behind, but work in model systems such asthe dimorphic bacterium Caulobacter crescentus isunveiling at least some of the regulatory strategiesemployed by prokaryotes to guide this essentialprocess. Most recently, Holtzendorff et al. [2] reportthat levels of two Caulobacter transcription factorsoscillate in reciprocal fashion over the course of thecell cycle, comprising a regulatory circuit directinglandmark events of the cell cycle.Caulobacter are aquatic bacteria that progressthrough an invariant series of developmental eventseach cell cycle [3] (Figure 1A). The ‘swarmer’ cell, amotile stage equipped to seek out nutrients, isanalogous to the G1 stage of the eukaryotic cell cyclein that chromosome replication is blocked (Figure 1A).The eventual transition into S phase — chromosomereplication — is marked by the swarmer cell sheddingits polar flagellum and replacing it with an adhesivestalk. As the sessile stalked cell elongates, a newflagellum is synthesized at the unoccupied cell pole.This event depends on ongoing DNA replication,through a poorly understood checkpoint mechanism[4]. Subsequent cell division also depends on completion of DNA replication and proper segregation of thedaughter chromosomes [4]. Until the work of Holtzendorff et al. [2], the CtrA transcription factor was the only known ‘master regulator’of the Caulobacter cell cycle. CtrA controls initiation ofDNA replication through binding to the origin [5], anddirectly controls the expression of at least 95 cell-cycleregulated genes involved in diverse processes, including cell division, flagellar motility, adhesion and DNAmethylation [6]. CtrA is subject to multiple forms of regulation. Expression of ctrA is cell-cycle regulated, andthe activity of CtrA depends on its phosphorylation by
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