eResearch Activity 3

How Do a Hundred Cells Divide at Once?

Dividing the cell is a complicated business, even for a relatively simple bacterium such as Escherichia coli. Forming two functional daughter cells requires intricate coordination of the DNA replication and cell growth, culminating in septation at the Z-ring (see Figures 3.23 and 3.25). Now suppose you have to form a hundred cells at once. How would that ever work?

The actinomycete Streptomyces venezuelae grows as a filament (or hypha), copying its DNA many times and extending without cell division. In a sporogenic (spore-forming) hypha, a molecular regulator promotes simultaneous septation at dozens of Z-rings. When FtsZ is labeled with a fluorophore, the fluorescence generates a dramatic ladder-like pattern called a “Z-ladder” (Fig. ERA 3.1A ). The Z-ladder enables a filament to divide and develop into numerous spores, which are carried off on air currents to colonize a new location. Streptomyces species are important to understand because they give us many antibiotics, such as streptomycin for tuberculosis and tetracycline for acne and respiratory bacterial infections.

A. S. venezuelae

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A fluorescent micrograph of a sporogenic hyphae of S. venezuelae. The hyphae is shown as a 45 micrometer line and a 15 micrometer line composed of Z rings. Both lines are long and fairly straight.

B. S. venezuelae ΔsepH

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A fluorescent micrograph of a sporogenic hyphae of a mutated S. venezuelae. The strain has had the s e p H gene deleted. Instead of long, independent hyphaes, this cell line has a cluster of line fragments that are approximately 20 micrometers long.

How do Streptomyces hyphae manage to coordinate a hundred cell divisions? Susan Schlimpert, at the John Innes Centre in Norwich, England, investigates the mechanisms of cell division in Streptomyces venezuelae, a model actinomycete (Fig. ERA 3.2A). A key cell division regulator was revealed by her postdoctoral fellow Félix Ramos-León and her research assistant Matthew Bush (Fig. ERA 3.2B).

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A photo of Susan Schlimpert. She has short blonde hair and a dark red shirt on. She appears in a lab setting.

A photo of Felix Ramos Leon and Matthew Bush. Felix has dark hair, a goatee and a dark orange shirt. Matthew has dark hair, glasses, a goatee and a dark plaid shirt.

Bush identified sepH as a gene of unknown function in the Streptomyces genome and showed that its expression is controlled by a major sporulation regulator. First, Ramos-León and colleagues investigated the SepH gene product by constructing a gene fusion to the gene encoding the fluorescent protein YPet. (Gene fusions for fluorescent proteins are discussed in Chapter 2.) The resulting fusion gene, sepH-ypet, encodes a protein that functions as SepH but also fluoresces as YPet by absorption of green light with emission of yellow light. In microscopy of S. venezuelae carrying the sepH-ypet gene, we see yellow fluorescence at the position of Z-rings. Thus, SepH has an appropriate location for a protein that assists FtsZ function.

Next, the researchers observed a strain of S. venezuelae that expresses FtsZ fused to YPet, so that the ladder of Z-rings fluoresces. The FtsZ-YPet fluorescence was observed in two different strains, either expressing sepH (Fig. ERA 3.1A) or deleted for sepH (ΔsepH) (Fig. ERA 3.1B ). For each experiment, a movie was made showing the coordinated development of the Z-ladder and sporulation. In the ΔsepH strain (a strain lacking sepH), sporulation becomes disorganized, causing cells to bulge into distorted forms and even to form filament branches. While some sporulation occurs, the process is greatly disrupted. Thus sepH was confirmed as a gene whose protein product organizes septation and sporulation in S. venezuelae.

Ramos-León further revealed that SepH stimulates the GTPase activity of FtsZ, a common mechanism of molecular regulation. A SepH homolog was found in the genus Mycobacterium, a group of Actinobacteria that includes the famous pathogen Mycobacterium tuberculosis. In pathogenic mycobacteria, SepH assists cell division without sporulation. Thus, discovery of this cell division protein advances our understanding of both an important drug producer and a major pathogen whose mechanisms may lead to novel antibiotic therapies.

Further Exploration

What experiments might you perform to show whether SepH interacts directly with the Z-ladder by binding to FtsZ? What other kinds of proteins might be needed?

Ramos-León, Félix, Matthew J. Bush, Joseph W. Sallmen, Govind Chandra, Jake Richardson, et al. 2021. A conserved cell division protein directly regulates FtsZ dynamics in filamentous and unicellular actinobacteria. Elife 10:e63387.