Week 9 HW: Cell-Free Systems
Homework Part A: General and Lecturer-Specific Questions
General homework questions
1. Explain the main advantages of cell-free protein synthesis over traditional in vivo methods, specifically in terms of flexibility and control over experimental variables. Name at least two cases where cell-free expression is more beneficial than cell production.
Cell-free protein synthesis (CFPS) offers significant advantages over traditional in vivo expression systems, primarily due to its flexibility and precise control over experimental conditions. Because CFPS operates in an open environment without living cells, researchers can directly manipulate the concentrations of DNA templates, ions, cofactors, and other components in real time. This eliminates constraints associated with cellular viability, such as toxicity or metabolic burden. As a result, CFPS is particularly advantageous for the production of proteins that are toxic to host cells, such as antimicrobial peptides or pore-forming proteins. Additionally, CFPS enables rapid prototyping of genetic constructs, making it highly suitable for applications like synthetic biology circuit testing, where speed and iterative design are essential.
2. Describe the main components of a cell-free expression system and explain the role of each component.
A cell-free expression system consists of several essential components that collectively replicate the molecular machinery of protein synthesis. The core component is the cell extract (lysate), which contains ribosomes, transfer RNAs (tRNAs), aminoacyl-tRNA synthetases, and various translation factors required for protein assembly. A DNA or messenger RNA (mRNA) template provides the genetic instructions encoding the target protein. Amino acids serve as the building blocks for protein synthesis, while an energy system—typically composed of ATP, GTP, and associated regeneration pathways—fuels transcription and translation processes. Additionally, salts and cofactors such as magnesium and potassium ions are necessary to maintain proper structural and functional conditions for enzymatic activity. When DNA is used as a template, transcriptional enzymes such as T7 RNA polymerase are also included to generate mRNA.
3. Why is energy provision regeneration critical in cell-free systems? Describe a method you could use to ensure continuous ATP supply in your cell-free experiment.
Energy regeneration is critical in CFPS because protein synthesis is an energy-intensive process that rapidly consumes ATP and GTP. Without a continuous supply of energy, translation halts prematurely, leading to low protein yields. To address this limitation, CFPS systems incorporate energy regeneration mechanisms that recycle ADP into ATP.
One commonly used method that I could use involves phosphoenolpyruvate (PEP) in combination with pyruvate kinase, which efficiently regenerates ATP during the reaction. Alternative systems, such as creatine phosphate with creatine kinase or glucose-based metabolic pathways, can also be employed depending on the desired duration and efficiency of protein production. These strategies extend reaction lifetimes and significantly improve overall protein yield.
4. Compare prokaryotic versus eukaryotic cell-free expression systems. Choose a protein to produce in each system and explain why.
Prokaryotic and eukaryotic CFPS systems differ in complexity, cost, and functional capabilities. Prokaryotic systems, such as those derived from Escherichia coli, are widely used due to their simplicity, high protein yield, and cost-effectiveness. However, they lack the machinery required for many post-translational modifications. These systems are well suited for expressing proteins that do not require complex folding or modifications, such as fluorescent reporters like GFP or metabolic enzymes.
In contrast, eukaryotic CFPS systems, including wheat germ or rabbit reticulocyte extracts, provide a more physiologically relevant environment that supports proper folding, disulfide bond formation, and certain post-translational modifications. Consequently, they are more appropriate for producing complex proteins such as human hormones or antibodies, where structural accuracy is critical for functionality.
5. How would you design a cell-free experiment to optimize the expression of a membrane protein? Discuss the challenges and how you would address them in your setup.
The expression of membrane proteins in CFPS systems presents unique challenges due to their hydrophobic nature and dependence on lipid environments for proper folding and stability. These proteins are prone to aggregation when synthesized in aqueous conditions. To overcome these challenges, CFPS reactions can be supplemented with membrane-mimicking systems such as liposomes, nanodiscs, or mild detergents that facilitate proper insertion and stabilization of the protein.
Additionally, molecular chaperones may be included to assist in correct folding. Careful optimization of ionic conditions, particularly magnesium and potassium concentrations, as well as modulation of expression rates, can further enhance protein quality. These strategies collectively create a suitable environment for functional membrane protein production.
6. Imagine you observe a low yield of your target protein in a cell-free system. Describe three possible reasons for this and suggest a troubleshooting strategy for each.
Low protein yield in CFPS systems can arise from several factors and one common issue is inefficient transcription or translation, which may result from weak promoters, suboptimal ribosome binding sites, or degraded DNA templates. This can be addressed by optimizing genetic elements, increasing template concentration, or ensuring DNA integrity.
A second factor is insufficient energy supply; rapid depletion of ATP can prematurely terminate protein synthesis. Implementing or optimizing an energy regeneration system can significantly improve yields. A third potential cause is protein misfolding or degradation, often due to the absence of proper folding conditions or the presence of proteases in the extract. This can be mitigated by adding molecular chaperones, reducing reaction temperature, or incorporating protease inhibitors. Systematic optimization of these parameters is essential to achieve efficient and reliable protein production.
Homework Question from Kate Adamala
1. Pick a function and describe it.
a) What would your synthetic cell do? What is the input and what is the output?
The synthetic minimal cell is designed to function as a biosensor and detoxification system for mercury contamination in aqueous environments. The input is the presence of mercury ions (Hg²⁺), which are detected by a mercury-responsive regulatory element inside the synthetic cell. In response, the system activates gene expression. The output consists of two components: (i) the production of a fluorescent reporter protein (GFP), which enables detection, and (ii) the enzymatic conversion of Hg²⁺ into elemental mercury (Hg⁰), a less toxic and more diffusible form.
b) Could this function be realized by cell-free Tx/Tl alone, without encapsulation?
No. Without encapsulation, the system would lack spatial organization and controlled interaction with the environment. The components responsible for sensing and response would diffuse freely, reducing efficiency and eliminating the ability to function as a defined, cell-like unit. Encapsulation is essential to maintain compartmentalization and regulate the exchange of molecules.
c) Could this function be realized by genetically modified natural cell?
Yes, this function could be implemented in genetically modified bacteria carrying mercury-resistance operons. However, such approaches involve the use of living genetically modified organisms, which raises biosafety and regulatory concerns. In contrast, synthetic minimal cells provide a non-living, modular alternative that allows precise control over system components and avoids environmental risks associated with engineered cells.
d) Describe the desired outcome of your synthetic cell operation.
The desired outcome is that, in the presence of mercury, the synthetic minimal cell simultaneously detects, reports, and detoxifies the contaminant. This results in both a measurable fluorescent signal and a reduction in mercury toxicity, enabling combined environmental sensing and remediation.
2. Design all components that would need to be part of your synthetic cell.
a) What would be the membrane made of?
The membrane would consist of a lipid bilayer composed of phospholipids such as POPC combined with cholesterol. This composition provides structural stability, appropriate fluidity, and controlled permeability, mimicking natural biological membranes.
b) What would you encapsulate inside? Enzymes, small molecules.
The synthetic cell would encapsulate a complete cell-free transcription/translation (Tx/Tl) system, including ribosomes, tRNAs, enzymes, amino acids, nucleotides, and cofactors. Additionally, it would contain the DNA encoding the mercury-responsive genetic circuit, an energy regeneration system (e.g., phosphoenolpyruvate-based), and all necessary components for protein synthesis, including the fluorescent reporter and detoxification enzymes.
c) Which organism your Tx/Tl system will come from? Is bacterial OK, or do you need a mammalian system for some reason? (hint: for example, if you want to use small molecule modulated promotors, like Tet-ON, you need mammalian)
The Tx/Tl system would be derived from bacterial extracts (Escherichia coli), as this system is efficient, cost-effective and compatible with the mercury-responsive regulatory elements used in the design.
Since the system does not require complex post-translational modifications, a prokaryotic expression system is sufficient.
d) How will your synthetic cell communicate with the environment? (hint: are substrates permeable? or do you need to express the membrane channel?)
Communication with the environment will be achieved through a combination of membrane permeability and specific transport mechanisms. Mercury ions (Hg²⁺), which are not readily permeable through lipid membranes, will enter the synthetic cell via membrane transport proteins such as MerT or MerP. Once inside, they activate the regulatory system. The detoxified product (Hg⁰) is more hydrophobic and can diffuse out of the membrane. The fluorescent signal remains inside the vesicle and can be detected externally using appropriate instrumentation.
3. Experimental details
a) List all lipids and genes. (bonus: find the specific genes; for example, instead of just saying “small molecule membrane channel” pick the actual gene.)
Lipids:
POPC (phosphatidylcholine)
Cholesterol
Genes:
merR (mercury-responsive transcriptional regulator)
merT and merP (mercury transport proteins)
merA (mercury reductase enzyme)
gfp (fluorescent reporter under mercury-inducible promoter)
Additional components:
Bacterial cell-free Tx/Tl system (E. coli extract)
Energy regeneration system (e.g., PEP + pyruvate kinase)
b) How will you measure the function of your system?
The function of the system will be evaluated using two complementary methods. First, fluorescence measurements will be used to quantify GFP expression as an indicator of mercury detection, using techniques such as plate readers or fluorescence microscopy. Second, chemical analysis of mercury transformation will be performed to confirm detoxification, using analytical methods such as atomic absorption spectroscopy to measure the conversion of Hg²⁺ to Hg⁰.
Homework question from Peter Nguyen
Freeze-dried cell-free systems can be incorporated into all kinds of materials as biological sensors or as inducible enzymes to modify the material itself or the surrounding environment. Choose one application field — Architecture, Textiles/Fashion, or Robotics — and propose an application using cell-free systems that are functionally integrated into the material. Answer each of these key questions for your proposal pitch:
Application field: Textiles in fashion.
Write a one-sentence summary pitch sentence describing your concept.
A smart textile incorporating freeze-dried cell-free systems that detects environmental pollutants and responds by producing visible color changes and neutralizing harmful compounds.
How will the idea work, in more detail? Write 3-4 sentences or more.
The proposed system consists of fabrics embedded with freeze-dried cell-free transcription/translation (Tx/Tl) reactions distributed within microcapsules integrated into the textile fibers. Upon exposure to environmental stimuli such as air pollutants (e.g., nitrogen oxides or volatile organic compounds), the system is activated by ambient moisture (humidity or sweat), which rehydrates the cell-free components. The embedded genetic circuits are designed to sense specific chemical signatures and trigger the expression of reporter proteins that produce visible color changes, allowing real-time detection. In addition, the system can express enzymes capable of partially degrading or neutralizing harmful compounds in the immediate surroundings. This creates a dual-function material that acts both as a biosensor and a localized remediation system.
What societal challenge or market need will this address?
Current monitoring systems are often centralized and do not provide individuals with real-time, localized information about their exposure. This smart textile addresses the need for personal, wearable environmental monitoring, empowering users to make informed decisions about their surroundings. Furthermore, integrating a remediation function adds value by not only detecting pollutants but also contributing to their reduction at a microenvironmental level. This concept is particularly relevant for urban populations, industrial workers and populations exposed to poor air quality.
How do you envision addressing the limitation of cell-free reactions (e.g., activation with water, stability, one-time use)?
One key limitation of cell-free systems is their dependence on hydration for activation. This can be addressed by designing the textile to utilize ambient humidity, sweat, or embedded hydrogel layers that retain moisture and enable controlled activation. Stability during storage can be improved through freeze-drying (lyophilization) combined with protective matrices such as sugars (trehalose), which preserve biological activity over extended periods. To address the one-time-use limitation, the textile can be engineered with replaceable or rechargeable patches containing the cell-free components.
Homework question from Ally Huang
Freeze-dried cell-free reactions have great potential in space, where resources are constrained. As described in my talk, the Genes in Space competition challenges students to consider how biotechnology, including cell-free reactions, can be used to solve biological problems encountered in space. While the competition is limited to only high school students, your assignment will be to develop your own mock Genes in Space proposal to practice thinking about biotech applications in space!
For this particular assignment, your proposal is required to incorporate the BioBits® cell-free protein expression system, but you may also use the other tools in the Genes in Space toolkit (the miniPCR® thermal cycler and the P51 Molecular Fluorescence Viewer). For more inspiration, check out https://www.genesinspace.org/ .
1. Provide background information that describes the space biology question or challenge you propose to address. Explain why this topic is significant for humanity, relevant for space exploration, and scientifically interesting. (Maximum 100 words)
Sustainable agriculture is essential for long-duration space missions, where food must be produced in controlled and resource-limited environments. Beneficial soil bacteria play a critical role in plant growth by promoting nutrient availability and stress resistance. However, microgravity and space radiation may alter bacterial gene expression and reduce their effectiveness. Understanding how plant growth–promoting bacteria respond to space conditions is therefore essential for developing reliable bioregenerative life-support systems. This topic is significant for enabling food production beyond Earth and scientifically interesting for studying microbial adaptation to extreme environments.
(It is a topic I always wanted to explore more about)
2. Name the molecular or genetic target that you propose to study. Examples of molecular targets include individual genes and proteins, DNA and RNA sequences, or broader -omics approaches. (Maximum 30 words)
Stress-response and plant-growth–related genes in Bacillus subtilis (spo0A, sigB and auxin-related pathways).
3. Describe how your molecular or genetic target relates to the space biology question or challenge your proposal addresses. (Maximum 100 words)
Bacillus subtilis is a model plant growth–promoting bacterium known for its resilience and ability to enhance plant health.
In space, environmental stressors such as microgravity and radiation may disrupt its gene expression, affecting its capacity to support plant growth. By analyzing stress-response and growth-related genes, whether beneficial bacterial functions are maintained under space conditions could be evaluated. This directly addresses the challenge of ensuring reliable microbial support systems for extraterrestrial agriculture.
4. Clearly state your hypothesis or research goal and explain the reasoning behind it. (Maximum 150 words)
The hypothesis is that space conditions, including microgravity and radiation, alter the expression of key stress-response and plant growth–promoting genes in Bacillus subtilis. Specifically, it is expected that stress-response genes such as sigB will be upregulated, while genes associated with plant growth promotion may be downregulated or dysregulated.
The research goal is to determine whether these changes can be detected using the BioBits® cell-free system as a rapid, portable diagnostic tool, and by linking gene expression outputs to fluorescent reporters, this system could enable real-time monitoring of microbial health and functionality in space. This approach supports the development of robust microbial systems for sustainable agriculture beyond Earth.
5. Outline your experimental plan - identify the sample(s) you will test in your experiment, including any necessary controls, the type of data or measurements that will be collected, etc. (Maximum 100 words)
Samples of Bacillus subtilis grown under simulated microgravity conditions will be compared to Earth controls. DNA or RNA will be extracted and amplified using the miniPCR. Target sequences will be introduced into the BioBits cell-free system with reporter constructs to measure gene expression via fluorescence. The P51 Molecular Fluorescence Viewer will be used to quantify signal intensity. Controls will include non-stressed bacteria and no-template reactions. Data will consist of fluorescence levels corresponding to gene activity, allowing comparison of stress-response and growth-related gene expression.
References.
Su, L., Wang, Y., Liu, J., & Zhang, X. (2023). Effects of short-term exposure to simulated microgravity on the physiology of Bacillus subtilis. Journal of Basic Microbiology. https://www.sciencedirect.com/org/science/article/pii/S0008416623000080
Morrison, M. D., Fajardo-Cavazos, P., & Nicholson, W. L. (2017). Cultivation in space flight produces minimal alterations in Bacillus subtilis physiology and spore formation. NPJ Microgravity, 3(1). https://pubmed.ncbi.nlm.nih.gov/28821547/