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7.20B: Caulobacter Differentiation

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    A Caulobacter is used for studying the regulation of the cell cycle, asymmetric cell division, and cellular differentiation.

    LEARNING OBJECTIVES

    Explain how caulobacter serve as a model organism

    Key Takeaways

    Key Points

    • The Caulobacter cell cycle regulatory system controls many modular subsystems that organize the progression of cell growth and reproduction.
    • The central feature of the cell cycle regulation is a cyclical genetic circuit—a cell cycle engine –- that is centered around the successive interactions of four master regulatory proteins: DnaA, GcrA, CtrA, and CcrM.
    • The interactions of four master regulatory proteins: DnaA, GcrA, CtrA, and CcrM directly control the timing of expression of over 200 genes. The four master regulatory proteins are synthesized and then eliminated from the cell one after the other over the course of the cell cycle.

    Key Terms

    • senescence: Ceasing to divide by mitosis because of shortening of telomeres or excessive DNA damage.
    • differentiation: In cellular differentiation, a less specialized cell becomes a more specialized cell.
    • modular: Consisting of separate modules; especially where each module performs or fulfills some specified function and could be replaced by a similar module for the same function, independently of the other modules.

    Caulobacter crescentus is a Gram-negative, oligotrophic bacterium widely distributed in fresh water lakes and streams. Caulobacter is an important model organism for studying the regulation of the cell cycle, asymmetric cell division, and cellular differentiation. Caulobacter daughter cells have two very different forms. One daughter is a mobile “swarmer” cell that has a single flagellum at one cell pole that provides swimming motility for chemotaxis. The other daughter, called the “stalked” cell has a tubular stalk structure protruding from one pole that has an adhesive holdfast material on its end, with which the stalked cell can adhere to surfaces. Swarmer cells differentiate into stalked cells after a short period of motility. Chromosome replication and cell division only occurs in the stalked cell stage. Its name is due to the fact that it forms a crescent shape; crescentin is a protein that imparts this shape.

    image
    Figure: Graphical representation of Caulobacter crescentus: Swarmer cells differentiate into stalked cells after a short period of motility.

    In the laboratory, researchers distinguish between C. crescentusstrain CB15 (the strain originally isolated from a freshwater lake) and NA1000 (the primary experimental strain). In strain NA1000, which was derived from CB15 in the 1970’s, the stalked and predivisional cells can be physically separated in the laboratory from new swarmer cells, while cell types from strain CB15 cannot be physically separated. The isolated swarmer cells can then be grown as a synchronized cell culture. Detailed study of the molecular development of these cells as they progress through the cell cycle has enabled researchers to understand Caulobacter cell cycle regulation in great detail. Due to this capacity to be physically synchronized, strain NA1000 has become the predominant experimental Caulobacter strain throughout the world. Additional phenotypic differences between the two strains have subsequently accumulated due to selective pressures on the NA1000 strain in the laboratory environment. The genetic basis of the phenotypic differences between the two strains results from coding, regulatory, and insertion/deletion polymorphisms at five chromosomal loci. “C. Crescentus” is synonymous with “Caulobacter Vibrioides. ”

    The Caulobacter cell cycle regulatory system controls many modular subsystems that organize the progression of cell growth and reproduction. A control system constructed using biochemical and genetic logic circuitry organizes the timing of initiation of each of these subsystems. The central feature of the cell cycle regulation is a cyclical genetic circuit—a cell cycle engine –- that is centered around the successive interactions of four master regulatory proteins: DnaA, GcrA, CtrA, and CcrM. These four proteins directly control the timing of expression of over 200 genes. The four master regulatory proteins are synthesized and then eliminated from the cell one after the other over the course of the cell cycle. Several additional cell signaling pathways are also essential to the proper functioning of this cell cycle engine.

    The principal role of these signaling pathways is to ensure reliable production and elimination of the CtrA protein from the cell at just the right times in the cell cycle. An essential feature of the Caulobacter cell cycle is that the chromosome is replicated once and only once per cell cycle. This is in contrast to the E. coli cell cycle where there can be overlapping rounds of chromosome replication simultaneously underway. The opposing roles of the Caulobacter DnaA and CtrA proteins are essential to the tight control of Caulobacter chromosome replication. The DnaA protein acts at the origin of replication to initiate the replication of the chromosome. The CtrA protein, in contrast, acts to block initiation of replication, so it must be removed from the cell before chromosome replication can begin. Multiple additional regulatory pathways integral to cell cycle regulation and involving both phospho signaling pathways and regulated control of protein proteolysis act to assure that DnaA and CtrA are present in the cell exactly when they are needed.

    Caulobacter was the first asymmetric bacterium shown to age. Reproductive senescence was measured as the decline in the number of progeny produced over time. A similar phenomenon has since been described in the bacterium Escherichia coli, which gives rise to morphologically similar daughter cells.

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