Week 9 HW: Cell Free Systems

Part 1: Cell-Free Protein Synthesis

Advantages of CFPS over in vivo expression

Cell-free protein synthesis (CFPS) removes the constraint of keeping a living cell alive. In a normal in vivo expression experiment, every design choice has to be compatible with growth, metabolism, membrane integrity, and host viability. In CFPS, the transcription and translation machinery is retained, but the cell itself is gone, so the reaction becomes an open biochemical system that can be directly tuned. DNA concentration, magnesium and potassium levels, redox state, chaperones, cofactors, detergents, lipids, noncanonical amino acids, and energy substrates can all be adjusted without worrying about whether the host will survive.

That open format gives two major advantages.

1. Rapid prototyping. I can test many DNA templates, promoter or RBS designs, or reaction conditions in parallel in a few hours, instead of building and transforming strains.
2. Tighter experimental control. Every important variable is directly set by the user rather than indirectly filtered through cellular regulation. If translation drops, I can alter Mg²+ or template concentration immediately. If a membrane protein aggregates, I can add nanodiscs or detergent directly to the reaction.

Cases where CFPS is especially beneficial

Main components of a CFPS system

A typical cell-free expression reaction has several core parts:

In short, CFPS reconstitutes the minimum biochemical environment needed for transcription and translation, then tunes that environment for the target protein or circuit.

Why energy regeneration is critical

Protein synthesis is extremely energy-intensive. ATP is required for tRNA charging and many upstream metabolic steps; GTP is consumed during translation initiation, elongation, and translocation. In a closed reaction, the energy pool is depleted quickly, and inhibitory byproducts such as inorganic phosphate accumulate. If ATP collapses, transcription slows, translation stalls, and yield drops sharply even if all other components are present.

A common practical solution is a phosphoenolpyruvate (PEP) + pyruvate kinase regeneration system. ATP is consumed during the reaction and converted to ADP. Pyruvate kinase then transfers the high-energy phosphate from PEP back onto ADP, regenerating ATP continuously. This setup is simple, fast, and effective for short to medium CFPS reactions.

In my experiment I would pair PEP regeneration with optimization of magnesium and phosphate balance, because even a good energy donor can fail if phosphate buildup poisons the reaction. For longer reactions I would also consider slower-burning substrates such as 3-phosphoglycerate, glucose, or maltodextrin, which often improve longevity by releasing energy more gradually.

Prokaryotic vs. eukaryotic CFPS

A prokaryotic system is best when the goal is speed, low cost, and high yield for proteins that do not require elaborate post-translational processing. To produce sfGFP I would choose an E. coli CFPS system: sfGFP folds well in bacterial conditions, does not need glycosylation, can be produced quickly at high yield, and gives a direct fluorescent readout of productive expression.

A eukaryotic system is preferable when the protein requires a eukaryotic folding environment, disulfide bond formation, microsomal insertion, or other processing steps. To produce human erythropoietin (EPO) I would choose a mammalian or insect-derived CFPS system: EPO is a secreted human glycoprotein whose activity and stability depend on proper eukaryotic folding and post-translational processing. An E. coli lysate could make the polypeptide but would be much less likely to produce a properly folded, functional therapeutic-like product.

Designing a CFPS experiment for a membrane protein

I would design the experiment around co-translational insertion into a membrane mimic, rather than expressing the protein into free solution and hoping it folds afterward. As a concrete example, I would use an E. coli CFPS system to express the bacterial potassium channel KcsA with a C-terminal GFP tag for rapid screening. The reaction would include preassembled nanodiscs made from MSP1D1 scaffold protein and a POPC:POPG lipid mixture, because KcsA is far more likely to remain soluble and native-like if it inserts into a bilayer during translation.

The main challenges:

To address these, I would screen a matrix of conditions:

• Nanodiscs vs. small liposomes vs. mild detergents (DDM, LMNG).
• Low vs. moderate DNA concentration.
• 25, 30, and 37°C reaction temperatures.
• Magnesium concentration and potassium glutamate concentration.
• Optional chaperone supplementation (DnaK / DnaJ / GrpE).

I would measure three outputs: total protein made, soluble or membrane-associated fraction, and functional activity after reconstitution. Total yield can be checked by SDS-PAGE or in-gel GFP fluorescence. Membrane insertion can be assessed by co-migration with nanodisc fractions or flotation assays. Function can be tested with a potassium flux assay after purification or direct reconstitution. The best condition is not simply the one with the most protein, but the one that gives the highest amount of correctly inserted, functional channel.

Three reasons for low CFPS yield

1. Poor template design or template quality. A weak promoter, a poorly matched RBS, degraded DNA, or problematic secondary structure in the coding sequence can hurt both transcription and translation. Troubleshooting: check DNA quality, compare plasmid vs. linear template, redesign the 5′ UTR, and test a stronger promoter or codon-optimized construct.

2. Incorrect reaction chemistry. CFPS depends sensitively on magnesium, potassium, pH, and energy balance. A reaction that is slightly off can collapse even with all components present. Troubleshooting: run a small DOE varying Mg²+, K+, DNA concentration, and energy substrate, while using a known positive control (e.g. sfGFP) to determine whether the issue is the reaction mixture or the target itself.

3. Protein instability, aggregation, or degradation. Some proteins fold poorly, are protease-sensitive, or precipitate as they are made. Troubleshooting: lower reaction temperature, shorten reaction time, add chaperones, add protease inhibitors, or include membrane mimics or redox helpers if the target is a membrane protein or disulfide-rich protein.

Low yield is usually not caused by one single factor. In practice, I would troubleshoot in the order template quality → reaction chemistry → protein-specific folding issues, because that sequence separates general reaction failure from target-specific failure.

Part 2: Design of a Useful Synthetic Minimal Cell

1. Pick a function and describe it.

I would design a synthetic minimal cell (SMC) that senses theophylline and, in response, activates a nearby engineered probiotic bacterium. The idea is to convert a small molecule that the bacterium does not naturally monitor into a standard bacterial induction signal.

This function could not be realized by cell-free Tx/Tl alone without encapsulation. If IPTG were simply mixed into a bulk cell-free reaction, it would diffuse directly to the bacteria and there would be no gated actuator step. The membrane compartment is what lets the SMC store the output signal until the input molecule triggers pore formation.

It could be realized by a genetically modified natural cell, but that would require engineering a living probiotic to directly sense theophylline and carry the entire logic internally. The synthetic-cell version is more modular: the same probiotic responder could be paired with many different SMC sensors just by swapping the sensing module.

The desired outcome is that the probiotic turns on only when theophylline is present, giving an external chemical control knob over bacterial behavior without permanently hard-wiring the sensing logic into the living cell.

2. Components

I would use a bacterial Tx/Tl system rather than a mammalian one, because the key regulatory element here is a small-molecule riboswitch and the output is just pore formation and inducer release. No mammalian glycosylation or nuclear machinery is needed.

The SMC communicates with the environment in two steps:

1. Theophylline diffuses across the vesicle membrane and binds the riboswitch, turning on pore synthesis.
2. Alpha-hemolysin inserts into the vesicle membrane and releases encapsulated IPTG, which then diffuses to the surrounding probiotic cells and activates their lac-regulated gene circuit.

3. Experimental details

Lipids and genes

• Lipids: POPC, cholesterol, DSPE-PEG2000.
• Tx/Tl system: E. coli S30 extract or PURE.
• Energy system: 3-phosphoglycerate or PEP-based ATP regeneration.
• Synthetic-cell gene: Staphylococcus aureus hla encoding alpha-hemolysin, controlled by a theophylline riboswitch.
• Encapsulated small-molecule cargo: IPTG.
• Responder-cell genes: constitutive lacI plus sfGFP under PlacUV5 or Ptac in E. coli Nissle 1917.

Measurement strategy

I would measure function primarily through the GFP output of the responder bacteria. In the presence of theophylline, the SMC should synthesize alpha-hemolysin, release IPTG, and induce bacterial GFP. The cleanest readout would be flow cytometry or plate-reader fluorescence of the E. coli Nissle reporter strain.

Key controls:

• No theophylline.
• No hla DNA.
• SMCs without encapsulated IPTG.
• Responder bacteria without the lac-regulated reporter.

If needed, IPTG release could also be confirmed indirectly by comparing fluorescence kinetics, or directly by chemical assay of the supernatant.

Part 3: Freeze-Dried Cell-Free Systems in Materials

One-sentence pitch

I propose a soft-robotic skin with embedded freeze-dried cell-free microcapsules that detect damage, generate a visible warning signal, and locally produce a crosslinking enzyme to help seal small tears.

How it works

The robotic skin would contain patterned microcapsules loaded with freeze-dried cell-free reactions, a DNA template for a visible chromoprotein, and a DNA template for microbial transglutaminase. These capsules would be embedded inside an elastomer layer that also contains a thin repair hydrogel rich in crosslinkable residues. When the skin is punctured or torn, a built-in water reservoir or ambient moisture rehydrates the damaged region and activates the local cell-free reactions. The chromoprotein marks the damaged area for easy inspection, while transglutaminase crosslinks the repair layer and helps slow crack growth or fluid leakage long enough for replacement.

Societal challenge / market need

Soft robots are increasingly used in medical devices, warehouse automation, and search-and-rescue environments, but their compliant materials are vulnerable to small tears, abrasion, and puncture. Today, many failures are only discovered after performance drops or a leak becomes severe. A self-reporting, partially self-sealing skin would reduce downtime, improve safety, and make soft robots more practical in environments where immediate maintenance is difficult.

Addressing CFPS limitations

I would address activation and stability by storing the reactions in trehalose-stabilized, vacuum-sealed microcapsules laminated inside the material until damage occurs. Water-triggering is actually useful here, because damage can be coupled to capsule rupture or exposure to a local hydration layer. The one-time-use limitation can be handled by making the sensing-and-repair elements modular and replaceable, like sacrificial patches in high-strain regions. For long shelf life, the material would use oxygen and moisture barrier films so the cell-free modules stay dormant until needed.

Part 4: Mock Genes in Space Proposal

1. Background

Long-duration missions may depend on dried DNA templates for on-demand production of medicines, enzymes, and diagnostics. Space radiation and temperature cycling could damage these templates and reduce the reliability of cell-free manufacturing. I want to test how well lightweight shielding preserves the functional expression capacity of stored DNA. This matters because future crews will need compact, stable biotechnology systems far from Earth, and it is scientifically interesting because it directly connects the space environment to the survival of usable genetic information.

2. Molecular or genetic target

Plasmid DNA encoding sfGFP under a T7 promoter, plus the T7-promoter-to-sfGFP junction as a PCR integrity marker.

3. How the target relates to the challenge

If spaceflight damages the promoter or coding sequence, BioBits should produce less GFP even when the same amount of DNA is added. Measuring fluorescence therefore converts DNA integrity into a simple functional readout. By comparing shielded and unshielded templates, I can test whether stored genetic instructions remain usable for future in-space biomanufacturing and biosensing.

4. Hypothesis

DNA stored behind lightweight, hydrogen-rich shielding will retain higher functional expression capacity than unshielded DNA after space exposure.

The goal is to compare practical storage strategies for preserving genetic templates that could later be used in cell-free systems aboard spacecraft. This hypothesis is based on the fact that ionizing radiation causes strand breaks and base damage, while hydrogen-rich materials can reduce secondary particle damage more effectively than many denser materials. A functional BioBits readout is especially useful because a template may still be amplifiable by PCR yet perform poorly in transcription or translation.

5. Experimental plan

I would test freeze-dried plasmid aliquots stored in three conditions: unshielded, polyethylene-shielded, and aluminum-shielded, with matched ground controls.

Fresh plasmid serves as a positive control; no-DNA reactions serve as negative controls.