Week 9: Cell-Free Systems

Cell-Free Systems Homework

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 major advantages over traditional in vivo expression because it removes the constraints imposed by maintaining living cells. Since the reaction occurs in vitro, the experimenter has much tighter control over variables such as DNA concentration, energy source, salts, cofactors, reaction timing, and additives. This makes CFPS especially useful for rapid prototyping, because gene circuits or expression constructs can be tested directly without cloning into cells and waiting for growth. It is also easier to study toxic proteins or unstable pathways in CFPS, since there is no living host whose growth is harmed by the product. In addition, freeze-dried cell-free systems are portable, low-maintenance, and can be deployed with minimal equipment, making them well suited for low-resource settings and space applications. This was demonstrated in the BioBits study aboard the ISS, where freeze-dried cell-free reactions were rehydrated and used to express aptamers and fluorescent proteins under microgravity conditions.

Two cases where cell-free expression is more beneficial than cell production are:

• Point-of-need biosensing or diagnostics, where portability and rapid response are more important than long-term cell growth.
• Expression of toxic or burdensome proteins, where the product would damage or slow the growth of living cells.

2. Describe the main components of a cell-free expression system and explain the role of each component.

A cell-free expression system contains the molecular machinery needed for transcription and translation outside of living cells. One major component is the cell extract, which provides ribosomes, tRNAs, translation factors, metabolic enzymes, and in some cases RNA polymerase. Another component is the DNA template, which encodes the gene or circuit to be expressed. The system also includes nucleotides for transcription, amino acids for translation, and salts/cofactors such as magnesium and potassium that support enzyme activity and ribosome function.

A critical part of the mixture is the energy system, which regenerates ATP and other high-energy molecules needed to power transcription and translation. Many systems also include supplements such as folinic acid, tRNA mixtures, cofactors, and reducing agents to improve expression efficiency. In the BioBits ISS paper, the reaction mixture included ATP, GTP/UTP/CTP, amino acids, potassium glutamate, ammonium glutamate, magnesium glutamate, NAD, CoA, spermidine, putrescine, phosphoenolpyruvate, and cell extract, all of which helped support robust gene expression in vitro.

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 cell-free systems because transcription and translation consume large amounts of ATP and GTP. Unlike living cells, CFPS reactions do not have intact metabolism to continuously replenish these molecules, so without an energy-regeneration strategy the reaction would stop quickly. Sustained protein synthesis therefore depends on including a substrate that can support ATP regeneration over time.

One common method is to include phosphoenolpyruvate (PEP) as an energy source. In crude extract systems, endogenous enzymes can use PEP to regenerate ATP and maintain the reaction for longer periods. The BioBits formulation used aboard the ISS included 33 mM phosphoenolpyruvate, showing exactly this kind of strategy for sustaining cell-free transcription and translation. In my own experiment, I could use a PEP-based energy system and compare protein yield over time to confirm that the reaction remains active.

4. Compare prokaryotic versus eukaryotic cell-free expression systems. Choose a protein to produce in each system and explain why.

A prokaryotic cell-free system is usually faster, cheaper, and easier to optimize than a eukaryotic system. It is well suited for expressing bacterial proteins, fluorescent reporters, enzymes, or genetic circuits that do not require complex post-translational modifications. For example, eGFP is a good protein to produce in a prokaryotic CFPS system because it folds relatively well, is easy to detect, and does not require glycosylation or other advanced processing.

A eukaryotic cell-free system is better for proteins that depend on more complex folding environments or post-translational modifications such as disulfide bonding, glycosylation, or membrane insertion. For example, a human cytokine or secreted antibody fragment would be a better candidate for a eukaryotic system, because bacterial extracts often cannot reproduce the same maturation steps needed for proper activity. In short, prokaryotic systems are excellent for speed and simplicity, while eukaryotic systems are preferable when the target protein requires more biologically complex processing.

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.

Membrane proteins are difficult to express in cell-free systems because they tend to misfold, aggregate, or precipitate when no membrane-like environment is available. To optimize expression, I would design a screen in which the same membrane-protein DNA template is tested across multiple reaction conditions that vary membrane mimics, detergent concentration, magnesium concentration, and temperature. A key addition would be some form of membrane support, such as liposomes, nanodiscs, or mild detergents, so that the newly synthesized protein has a hydrophobic environment into which it can insert.

I would also monitor both total expression and soluble/functional expression, since a high total yield is not useful if the protein is aggregated. Suitable readouts might include SDS-PAGE, fluorescence tagging, or an activity assay if the membrane protein has measurable function. A lower reaction temperature and codon-optimized DNA could also help improve folding. Overall, the main challenge is not just producing the protein, but producing it in a membrane-compatible state that preserves structure and activity.

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.

One possible reason is that the DNA template concentration or quality is poor. If the template is degraded, contaminated, or present at too low a concentration, transcription may be inefficient. A troubleshooting strategy would be to verify the DNA on a gel, re-purify it, and test a range of template concentrations.

A second possible reason is that the reaction chemistry is suboptimal, for example incorrect magnesium concentration, depleted energy source, or poor buffer balance. Since CFPS is very sensitive to reaction composition, I would troubleshoot by running a small matrix of conditions and adjusting magnesium, potassium, and energy-substrate levels systematically.

A third possible reason is that the target protein itself is difficult to express or fold. This is especially likely for large, toxic, disulfide-rich, or membrane proteins. In that case, I would try lowering the temperature, changing the reaction time, adding chaperones or membrane mimics, or switching to a different cell-free system better suited to the protein class.

Homework Question from Kate Adamala

Design an example of a useful synthetic minimal cell

Pick a function and describe it.

My synthetic minimal cell would function as a localized ammonia detoxification sensor-actuator for aquatic environments. Its purpose would be to detect elevated ammonia in contaminated water and respond by producing an enzyme-based output that helps convert ammonia into a less harmful form.

What would your synthetic cell do? What is the input and what is the output?

The input would be elevated environmental ammonia. The output would be expression of a reporter such as GFP together with an enzymatic detoxification module, for example glutamine synthetase activity coupled to ATP-dependent ammonia assimilation.

Could this function be realized by cell-free Tx/Tl alone, without encapsulation?

Not as effectively. Without encapsulation, the reaction would simply mix into the surrounding environment and lose the separation between sensing, response, and controlled release. Encapsulation makes the system behave more like an artificial cell rather than just a biochemical mixture.

Could this function be realized by genetically modified natural cell?

Yes, it could, but that would introduce issues of survival, biocontainment, ecological competition, and environmental release. A synthetic minimal cell would reduce those concerns by remaining non-living and function-limited.

Describe the desired outcome of your synthetic cell operation.

In the presence of elevated ammonia, the synthetic cell should produce a measurable fluorescent signal and activate a detoxification pathway, demonstrating both environmental sensing and a functional biochemical response.

Design all components that would need to be part of your synthetic cell.

The system would need:

• a lipid vesicle membrane
• a cell-free transcription/translation system
• an ammonia-responsive genetic control element
• a reporter gene such as GFP
• an output enzyme system relevant to ammonia capture or conversion
• ATP and metabolic cofactors
• permeable access for ammonia or a transport mechanism if needed

What would the membrane be made of?

The membrane could be made of POPC + cholesterol, similar to common synthetic-cell vesicle formulations, because this provides a reasonably stable phospholipid bilayer.

What would you encapsulate inside? Enzymes, small molecules.

Inside I would encapsulate:

• bacterial cell-free Tx/Tl extract
• amino acids, nucleotides, salts, and energy mix
• ATP-regeneration substrate
• plasmid DNA for the reporter and detoxification module
• glutamine synthetase or related enzymatic module if not produced in situ

Which organism would your Tx/Tl system come from?

A bacterial system would likely be sufficient, because the sensing and output logic do not require mammalian transcription factors or mammalian-specific signaling pathways. A bacterial extract would also be simpler and cheaper.

How will your synthetic cell communicate with the environment?

Ammonia is a small molecule and can cross membranes to some extent, so the input may not require a dedicated channel. If permeability is insufficient, I could incorporate a membrane pore such as alpha-hemolysin (aHL) to facilitate exchange.

Experimental details

Lipids: POPC, cholesterol
Genes: GFP reporter, alpha-hemolysin (aHL) if needed for permeability, and a detoxification-related gene such as glnA (glutamine synthetase)
How will you measure function?
I would measure:

• GFP fluorescence as the sensing readout
• ammonia concentration before and after treatment as the functional output
• optionally, vesicle integrity by microscopy or dye retention

Homework Question from Peter Nguyen

Write a one-sentence summary pitch sentence describing your concept.

I propose a freeze-dried cell-free textile patch that activates on contact with sweat and reports dehydration-related electrolyte imbalance through a visible fluorescent color change.

How will the idea work, in more detail?

The concept is a wearable textile patch containing freeze-dried cell-free reactions embedded into a layered fabric substrate. When activated by sweat, the patch rehydrates and the cell-free biosensor responds to a target chemical signature correlated with electrolyte loss or dehydration stress. The output could be a fluorescent or colorimetric signal visible to the wearer or captured by a phone camera. Because the system is cell-free, it avoids the maintenance and containment problems associated with living engineered cells while remaining lightweight and low-cost. The broader idea follows the same logic seen in portable cell-free diagnostics: on-demand activation, minimal equipment, and visible readout in low-resource conditions.

What societal challenge or market need will this address?

This could address the need for simple, wearable, low-cost monitoring tools for athletes, outdoor workers, military personnel, or people exposed to heat stress. A textile-integrated indicator could help users detect physiologically dangerous conditions earlier without needing batteries or complex electronics.

How do you envision addressing the limitation of cell-free reactions?

The main limitation is that freeze-dried cell-free systems are typically single-use and require rehydration. I would address this by designing the patch as a replaceable disposable insert within a reusable textile holder. Stability could be improved through lyophilization and protective packaging, and the system would be intentionally framed as an intermittent-use sensor rather than a permanent continuous monitor. The ISS BioBits paper is relevant here because it shows that freeze-dried cell-free systems can remain portable, stable, and functional in constrained environments while producing visually trackable fluorescent outputs.

Homework Question from Ally Huang

Background information

A major challenge in long-duration spaceflight is the need for lightweight, low-maintenance biological tools that can function without extensive laboratory infrastructure. Cell-free systems are promising because they do not require living cells, can be freeze-dried for storage, and can be activated on demand. This makes them relevant for space biology, where crew time, mass, stability, and biocontainment are all constrained. The ISS validation of BioBits showed that freeze-dried cell-free transcription, translation, and biosensing can function robustly in microgravity, making this platform scientifically interesting both for astronaut health monitoring and for biological analysis in future space missions.

Molecular or genetic target

A stress-responsive RNA or DNA target associated with microbial contamination in spacecraft water systems, detected using a sequence-specific toehold-switch or aptamer-based BioBits sensor.

Describe how your target relates to the challenge

A nucleic-acid target related to microbial contamination is directly relevant to astronaut safety because spacecraft currently rely heavily on delayed or ground-based monitoring of potentially contaminated samples. A cell-free biosensor that detects a specific microbial sequence would provide a simpler and more immediate onboard screening tool. This is scientifically interesting because it combines portable synthetic biology with spaceflight constraints, and it builds directly on the demonstrated ability of BioBits to express toehold-switch and aptamer-based biosensors aboard the ISS.

Hypothesis or research goal

My hypothesis is that a freeze-dried BioBits cell-free reaction can be used in space to detect a specific microbial nucleic-acid sequence relevant to spacecraft environmental monitoring and produce a visible fluorescence readout using the Genes in Space toolkit. The reasoning is that BioBits has already been shown to support transcription, translation, and biosensor function aboard the ISS, including toehold-switch-based detection of RNA targets and direct fluorescence readout with the Genes in Space Fluorescence Viewer. If a custom target sequence associated with a spacecraft waterborne microbe were inserted into a toehold-switch detection scheme, the same general architecture could be repurposed into a practical onboard monitoring assay. This would support faster environmental testing in space and reduce dependence on returning samples to Earth for analysis.

Experimental plan

I would prepare freeze-dried BioBits reactions containing a toehold-switch plasmid designed against a target microbial RNA sequence. Samples tested would include: correct target RNA, mismatched RNA control, no-target negative control, and blank reaction control. Reactions would be rehydrated and incubated using the BioBits workflow, then monitored with the P51 / Genes in Space fluorescence viewer. The main data collected would be fluorescence intensity over time and endpoint comparison between target and control conditions. A successful result would show fluorescence only in the presence of the correct target sequence, similar to the toehold-switch logic validated aboard the ISS.

References

• Kocalar, S., Miller, B. M., Huang, A., Gleason, E., Martin, K., Foley, K., Copeland, D. S., Jewett, M. C., Alvarez Saavedra, E., & Kraves, S. (2024). Validation of cell-free protein synthesis aboard the International Space Station. ACS Synthetic Biology, 13, 942–950. https://doi.org/10.1021/acssynbio.3c00733