Ultrastructure of Exospore Formation in Streptomyces Revealed by Cryo-Electron Tomography

Abstract

Many bacteria form spores in response to adverse environmental conditions. Several sporulation pathways have evolved independently and occur through distinctive mechanisms. Here, using cryo-electron tomography (cryo-ET), we examine all stages of growth and exospore formation in the model organism Streptomyces albus. Our data reveal the native ultrastructure of vegetative hyphae, including the likely structures of the polarisome and cytoskeletal filaments. In addition, we observed septal junctions in vegetative septa, predicted to be involved in protein and DNA translocation between neighboring cells. During sporulation, the cell envelope undergoes dramatic remodeling, including the formation of a spore wall and two protective proteinaceous layers. Mature spores reveal the presence of a continuous spore coat and an irregular rodlet sheet. Together, these results provide an unprecedented examination of the ultrastructure in Streptomyces and further our understanding of the structural complexity of exospore formation.

The Sporobiota of the Human Gut

“…The formation of the exospore in Streptomyces begins extracellularly, as nonbranching aerial hyphae form from the colony surface. 10,11 Myxospores are a third type of spore produced by Myxococcus xanthus, a Gram-negative bacterium from the Proteobacteria phylum. In M. xanthus, spores are formed by the rearrangement of the rod-shaped vegetative cell to a spherical spore, in response to starvation conditions.…”

Structural, Metabolic and Evolutionary Comparison of Bacterial Endospore and Exospore Formation

“…Immature spores remain associated with each other in the spore chain as they undergo maturation, likely held in place by the rodlet ultrastructure that forms a sheath on the surface of aerial hyphae. Maturation is accompanied by an increase in spore wall thickness ( Figure 2I ), presumably directed by MreB and other components of the spore wall synthesizing complex, consisting of proteins from the mre gene cluster along with RodZ, FtsI, and SCO3901 ( Heichlinger et al, 2011 ; Kleinschnitz et al, 2011 ; Girard et al, 2014 ; Sexton and Tocheva, 2020 ). In addition to increased thickness, the PG in the spore wall features an increase in 3–4 crosslinks and a decrease in 3–3 crosslinks ( van der Aart et al, 2018 ), and multiple layers are visible in the spore wall after spore maturation ( Sexton and Tocheva, 2020 ).…”

“…Maturation is accompanied by an increase in spore wall thickness ( Figure 2I ), presumably directed by MreB and other components of the spore wall synthesizing complex, consisting of proteins from the mre gene cluster along with RodZ, FtsI, and SCO3901 ( Heichlinger et al, 2011 ; Kleinschnitz et al, 2011 ; Girard et al, 2014 ; Sexton and Tocheva, 2020 ). In addition to increased thickness, the PG in the spore wall features an increase in 3–4 crosslinks and a decrease in 3–3 crosslinks ( van der Aart et al, 2018 ), and multiple layers are visible in the spore wall after spore maturation ( Sexton and Tocheva, 2020 ). It is unclear if the spore wall layers feature the same PG structure, and what the biological significance of modified PG crosslinking is in the spore wall.…”

“…On the other hand, the Actinobacterial exospore coat is characterized by a simpler architecture. For example, a thin spore coat has been observed on the surface of the spore wall of Streptomyces spores ( Sexton and Tocheva, 2020 ). However, the composition and function of this coat in spore survival remain to be characterized.…”

“…Finally, exospores are presumably released from the spore chain by mechanical means. As is the case with endospores, exospores remain dormant in the environment until favorable growth conditions are sensed, at which point they germinate and resume vegetative growth ( Sexton and Tocheva, 2020 ). Despite being a model system, less is known about the regulation and mechanics of sporulation in Streptomyces compared to Firmicutes.…”