Editing Caulobacter crescentus (section)

==Evolutionary conservation of the cell cycle control system==
The control circuitry that directs and paces ''Caulobacter'' cell cycle progression involves the entire cell operating as an integrated system. The control circuitry monitors the environment and the internal state of the cell, including the cell topology, as it orchestrates activation of cell cycle subsystems and ''Caulobacter crescentus'' asymmetric cell division. The proteins of the ''Caulobacter'' cell cycle control system and its internal organization are co-conserved across many alphaproteobacteria species, but there are great differences in the regulatory apparatus' functionality and peripheral connectivity to other cellular subsystems from species to species.<ref>{{cite journal|doi=10.1016/j.jmb.2011.02.041|pmid=21371478|pmc=3108490|title=The Architecture and Conservation Pattern of Whole-Cell Control Circuitry|journal=Journal of Molecular Biology|volume=409|issue=1|pages=28–35|year=2011|last1=McAdams|first1=Harley H.|last2=Shapiro|first2=Lucy}}</ref><ref>{{cite journal|doi=10.1186/1752-0509-4-52|pmid=20426835|pmc=2877005|title=The diversity and evolution of cell cycle regulation in alpha-proteobacteria: A comparative genomic analysis|journal=BMC Systems Biology|volume=4|pages=52|year=2010|last1=Brilli|first1=Matteo|last2=Fondi|first2=Marco|last3=Fani|first3=Renato|last4=Mengoni|first4=Alessio|last5=Ferri|first5=Lorenzo|last6=Bazzicalupo|first6=Marco|last7=Biondi|first7=Emanuele G. |doi-access=free }}</ref>  The ''Caulobacter'' cell cycle control system has been exquisitely optimized by evolutionary selection as a total system for robust operation in the face of internal [[stochastic]] noise and environmental uncertainty.

The bacterial cell's control system has a hierarchical organization.<ref>{{cite journal | last1 = McAdams | first1 = HH | last2 = Shapiro | first2 = L. |date=May 2011 | title = The architecture and conservation pattern of whole-cell control circuitry | journal = J Mol Biol | volume = 409 | issue = 1| pages = 28–35 | doi = 10.1016/j.jmb.2011.02.041 | pmid = 21371478 | pmc = 3108490 }}</ref> The signaling and the control subsystem interfaces with the environment by means of sensory modules largely located on the cell surface. The genetic network logic responds to signals received from the environment and from internal cell status sensors to adapt the cell to current conditions. A major function of the top level control is to ensure that the operations involved in the cell cycle occur in the proper temporal order. In ''Caulobacter'', this is accomplished by the genetic regulatory circuit composed of five master regulators and an associated phospho-signaling network. The phosphosignaling network monitors the state of progression of the cell cycle and plays an essential role in accomplishing asymmetric cell division. The cell cycle control system manages the time and place of the initiation of chromosome replication and [[cytokinesis]] as well as the development of [[polar organelle]]s. Underlying all these operations are the mechanisms for production of protein and structural components and energy production. The “housekeeping” metabolic and catabolic subsystems provide the energy and the molecular raw materials for protein synthesis, cell wall construction and other operations of the cell. The housekeeping functions are coupled bidirectionally to the cell cycle control system. However, they can adapt, somewhat independently of the cell cycle control logic, to changing composition and levels of the available nutrient sources.

The proteins of the ''Caulobacter'' cell cycle control system are widely co-conserved across the alphaproteobacteria, but the ultimate function of this regulatory system varies widely in different species. These evolutionary changes reflect enormous differences between the individual species in fitness strategies and ecological niches. For example, ''[[Agrobacterium tumefaciens]]'' is a plant pathogen, ''[[Brucella abortus]]'' is an animal pathogen, and ''[[Sinorhizobium meliloti]]'' is a soil bacterium that invades, and becomes a [[symbiont]] in, plant root nodules that fix nitrogen yet most of the proteins of the ''Caulobacter'' cell cycle control are also found in these species. The specific coupling between the protein components of the cell cycle control network and the downstream readout of the circuit differ from species to species. The pattern is that the internal functionality of the network circuitry is conserved, but the coupling at the “edges” of the regulatory apparatus to the proteins controlling specific cellular functions differs widely among the different species.