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The bacterial chromosome is both highly supercoiled and bound by an ensemble of proteins and RNA, causing the DNA to form a compact structure termed the nucleoid. The nucleoid serves to condense, protect, and control access to the bacterial chromosome through a variety of mechanisms that remain incompletely understood. The nucleoid is also a dynamic structure, able to change both in size and composition. The dynamic nature of the bacterial nucleoid is particularly apparent when studying the effects of various stresses on bacteria, which require cells to protect their DNA and alter patterns of transcription. Stresses can lead to large changes in the organization and composition of the nucleoid on timescales as short as a few minutes. Here, we summarize some of the recent advances in our understanding of how stress can alter the organization of bacterial chromosomes. 

Abstract The DNA-binding protein from starved cells (Dps) plays a crucial role in maintaining bacterial cell viability during periods of stress. Dps is a nucleoid-associated protein that interacts with DNA to create biomolecular condensates in live bacteria. Purified Dps protein can also rapidly form large complexes when combined with DNA in vitro. However, the mechanism that allows these complexes to nucleate on DNA remains unclear. Here, we examine how DNA topology influences the formation of Dps–DNA complexes. We find that DNA supercoils offer the most preferred template for the nucleation of condensed Dps structures. More generally, bridging contacts between different regions of DNA can facilitate the nucleation of condensed Dps structures. In contrast, Dps shows little affinity for stretched linear DNA before it is relaxed. Once DNA is condensed, Dps forms a stable complex that can form inter-strand contacts with nearby DNA, even without free Dps present in solution. Taken together, our results establish the important role played by bridging contacts between DNA strands in nucleating and stabilizing Dps complexes. 

Cutting-edge photonic devices frequently rely on microparticle components to focus and manipulate light. Conventional methods used to produce these microparticle components frequently offer limited control of their structural properties or require low-throughput nanofabrication of more complex structures. Here, we employ a synthetic biology approach to produce environmentally friendly, living microlenses with tunable structural properties. We engineeredEscherichia colibacteria to display the silica biomineralization enzyme silicatein from aquatic sea sponges. Our silicatein-expressing bacteria can self-assemble a shell of polysilicate “bioglass” around themselves. Remarkably, the polysilicate-encapsulated bacteria can focus light into intense nanojets that are nearly an order of magnitude brighter than unmodified bacteria. Polysilicate-encapsulated bacteria are metabolically active for up to 4 mo, potentially allowing them to sense and respond to stimuli over time. Our data demonstrate that synthetic biology offers a pathway for producing inexpensive and durable photonic components that exhibit unique optical properties. 

Abstract Recently, engineered bacterial cells have been shown to behave as optically-active photonic devices comparable to industrially fabricated microlenses¹. Bacterial cells can be encapsulated within a layer of polysilicate through surface display of the sea sponge enzyme silicatein, which mineralizes a polysilicate coating. The addition of this polysilicate layer significantly enhances the ability of these cells to guide, scatter, and focus light¹. However, this previous technique was limited to creating rod-shaped microlenses, which are not ideal for all applications. Here we expand upon this technology by engineering the shapes of silicatein-displaying bacterial cells. Through the overexpression of the genesbolA^2–5andsulA^6,7or through the use of the drug A22^8,9, we are able to alterEscherichia colicells from their characteristic rod-like shape to either spherical or filamentous forms. Round cells encapsulated in polysilicate were shown to scatter light more intensely and symmetrically than rod-shaped cells, while encapsulated filamentous cells were shown to guide light similarly to an optical fiber. This control over the size and shape of optically-active cells is a major advancement towards developing bio-engineered photonic devices such as nanophotonic waveguides, spherical microlens arrays, and advanced biosensors.