Developmental Regulation

Sporulation in Bacillus

Sporulation is the last-ditch response to starvation; it is suppressed until alternative responses prove inadequate.

Learning Objectives

Explain sporulation in Bacillus

Key Takeaways

Key Points

  • B. subtilis can divide symmetrically to make two daughter cells (binary fission), or asymmetrically, producing a single endospore that is resistant to environmental factors such as heat, desiccation, radiation, and chemical insult which can persist in the environment for long periods of time.
  • The process of endospore formation has profound morphological and physiological consequences: radical post-replicative remodelling of two progeny cells, accompanied eventually by cessation of metabolic activity in one daughter cell (the spore ) and death by lysis of the other (the ‘mother cell’).
  • Sporulation in B. subtilis is induced by starvation; the sporulation developmental program is not initiated immediately when growth slows due to nutrient limitation.

Key Terms

  • endospore: A dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum.
  • sporulation: The process of a bacterium becoming a spore.

Bacillus subtilis is a rod-shaped, Gram-postive bacteria that is naturally found in soil and vegetation. It is known for its ability to form a small, tough, protective, and metabolically dormant endospore. B. subtilis can divide symmetrically to make two daughter cells (binary fission), or asymmetrically, producing a single endospore that is resistant to environmental factors such as heat, desiccation, radiation, and chemical insult which can persist in the environment for long periods of time. The endospore is formed at times of nutritional stress, allowing the organism to persist in the environment until conditions become favourable. The process of endospore formation has profound morphological and physiological consequences: radical post-replicative remodeling of two progeny cells, accompanied eventually by cessation of metabolic activity in one daughter cell (the spore) and death by lysis of the other (the ‘mother cell’).

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B. subtillis: Colonies of B. subtilis grown on a culture dish in a molecular biology laboratory.

Although sporulation in B. subtilis is induced by starvation, the sporulation developmental program is not initiated immediately when growth slows due to nutrient limitation. A variety of alternative responses can occur:

  • The activation of flagellar motility to seek new food sources by chemotaxis
  • The production of antibiotics to destroy competing soil microbes
  • The secretion of hydrolytic enzymes to scavenge extracellular proteins and polysaccharides, or the induction of ‘competence’ for uptake of exogenous DNA for consumption, with the occasional side-effect that new genetic information is stably integrated.

Sporulation is a last-ditch response to starvation, and it is suppressed until alternative responses prove inadequate. Even then, certain conditions must be met, such as chromosome integrity, the state of chromosomal replication, and the functioning of the Krebs cycle.

Sporulation requires a great deal of time and energy, and it is essentially irreversible, making it crucial for a cell to monitor its surroundings efficiently and ensure that sporulation is embarked upon at only the most appropriate times. The wrong decision can be catastrophic: a vegetative cell will die if the conditions are too harsh, while bacteria-forming spores in an environment which is conducive to vegetative growth will be outcompeted. In short, initiation of sporulation is a very tightly regulated network with numerous checkpoints for efficient control.

Two transcriptional regulators, σH and Spo0A, play key roles in initiation of sporulation. Several additional proteins participate, mainly by controlling the accumulated concentration of Spo0A~P. Spo0A lies at the end of a series of inter-protein phosphotransfer reactions, Kin–Spo0F–Spo0B–Spo0A, termed as a ‘phosphorelay’.

Caulobacter Differentiation

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.

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

In the laboratory, researchers distinguish between C. crescentus strain 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.