week 9 HW: Cell-Free Systems

General Cell-Free Homework Questions

1. Main advantages of cell-free protein synthesis over in vivo methods

Cell-free systems offer direct access to the reaction environment — you can adjust pH, redox conditions, cofactor concentrations, and template DNA without having to engineer a living cell to tolerate those changes. You’re also not constrained by what the cell needs to survive; toxic proteins, non-natural amino acids, and unstable intermediates can all be produced because there’s no membrane to cross and no cellular fitness cost.

Two cases where cell-free beats cell-based production: (1) membrane proteins, which are toxic to host cells when overexpressed but can be synthesized directly into detergent micelles or liposomes in vitro; (2) rapid prototyping of genetic parts, where you want to test many regulatory sequences quickly without the cloning, transformation, and selection cycles required for in vivo work.

2. Main components of a cell-free expression system

• Cell extract — provides ribosomes, translation factors, RNA polymerase, chaperones, and metabolic enzymes. It’s the core catalytic machinery.
• DNA template — the gene of interest, typically under a strong promoter (T7 is common). Drives transcription.
• NTPs/amino acids — raw building blocks for RNA synthesis and translation.
• Energy source — ATP and GTP to power transcription, translation, and tRNA charging. Often supplied as phosphocreatine + creatine kinase, or similar regeneration system.
• Salts and buffer — Mg²⁺ and K⁺ concentrations are particularly critical for ribosome function and must be carefully titrated.

3. Energy regeneration in cell-free systems

Without continuous ATP regeneration, reactions stall within minutes as nucleotides are consumed. The most common solution is a coupled phosphate regeneration system: phosphocreatine is used as a high-energy phosphate donor, and creatine kinase regenerates ATP from ADP continuously. An alternative is a maltose/maltodextrin system, where glucose-1-phosphate derived from maltodextrin feeds directly into glycolytic ATP production — this gives a longer-lasting energy supply and avoids phosphate accumulation, which inhibits reactions at high concentrations.

4. Prokaryotic vs. eukaryotic cell-free systems

Prokaryotic systems (typically E. coli extract) are faster, cheaper, and higher-yield. They’re ideal for proteins that don’t require post-translational modifications. A good candidate would be T7 RNA polymerase — a bacterial protein, no glycosylation needed, benefits from high yield. Eukaryotic systems (wheat germ, rabbit reticulocyte, or HeLa extract) are slower and more expensive but contain the ER-derived vesicles and glycosylation machinery needed for complex proteins. A good candidate would be erythropoietin (EPO) — a human hormone that requires N-glycosylation for proper folding and biological activity, which a bacterial system cannot provide.

5. Cell-free expression of a membrane protein

Membrane proteins aggregate and crash out of solution when expressed without a lipid environment. The strategy is to include detergent micelles, nanodiscs, or liposomes directly in the reaction so the protein folds into a membrane-like environment co-translationally. You’d optimize detergent type and concentration empirically, likely starting with digitonin or DDM. Other variables to tune: Mg²⁺ concentration (affects ribosome processivity on difficult transmembrane segments), temperature (lower temps reduce aggregation), and supplementing with lipids matching the protein’s native membrane composition. Yield is assessed by western blot, and function by a binding or activity assay specific to the protein.

6. Troubleshooting low protein yield

Poor transcription — check that template DNA is clean and supercoiled (or linearized appropriately for T7), that the promoter sequence is correct, and that NTP concentrations are not depleted. Fix: run a separate transcription-only reaction and verify mRNA production by gel. Ribosome inhibition from Mg²⁺ imbalance — Mg²⁺ concentration is the single most sensitive variable in cell-free translation. Too high or too low kills yield. Fix: run a Mg²⁺ titration across a 1–2 mM range bracketing your current condition.

Protein degradation by extract proteases — some proteins are rapidly degraded post-synthesis. Fix: add protease inhibitor cocktail, or switch to a protease-reduced extract strain like E. coli BL21, which lacks the Lon and OmpT proteases.

Questions from Kate Adamala

1. Pick a function and describe it.

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

Act as a minimal artificial pancreatic beta cell. Input: elevated extracellular glucose. Output of the SMC: insulin. Output of the whole system: normalized blood glucose in surrounding environment.

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

No. Without encapsulation, insulin would diffuse freely regardless of glucose levels — there is no mechanism to gate release on a signal without a compartment.

• Could this function be realized by a genetically modified natural cell?

Yes — engineered beta cell therapies attempt exactly this. However, synthetic cells avoid immune rejection, don’t replicate uncontrollably, and are easier to tune and replace without genetic modification of a living cell.

• Describe the desired outcome of your synthetic cell operation.

In hyperglycemic conditions, the synthetic cell senses glucose and releases insulin.

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

a. What would the membrane be made of? POPC + cholesterol.

b. What would you encapsulate inside? Cell-free Tx/Tl system, pre-synthesized insulin, gene for aHL pore under control of a glucose-responsive riboswitch, glucokinase as intracellular glucose sensor.

c. Which organism will your Tx/Tl system come from? Bacterial, because glucose-responsive riboswitches function in bacterial transcription machinery and no mammalian post-translational modifications are required.

d. How will your synthetic cell communicate with the environment? Glucose is membrane-permeable and enters passively. Insulin is too large to cross unaided — release occurs via aHL pore expressed after glucose-triggered riboswitch activation.

3. Experimental details a. List all lipids and genes.

• Lipids: POPC, cholesterol
• Enzymes: bacterial cell-free Tx/Tl
• Genes: alpha-hemolysin (aHL) under glucose-responsive riboswitch, glucokinase
• Pre-encapsulated small molecule: insulin

b. How will you measure the function of your system?

• ELISA for insulin in supernatant across a range of glucose concentrations. Alternatively, use a fluorescent insulin analog and measure bulk release by fluorometry.

Questions from Peter Nguyen

Application field: Textiles/Fashion

One-sentence summary pitch:

A wound-responsive bandage textile embedded with freeze-dried cell-free systems that could detect bacterial infection biomarkers and produce antimicrobial peptides on-site in response.

How will the idea work?

The concept would involve weaving fibers containing freeze-dried cell-free Tx/Tl machinery loaded with a gene for an antimicrobial peptide (such as defensin) under the control of a promoter responsive to a bacterial quorum sensing molecule (e.g. AHL, acyl-homoserine lactone). When wound exudate rehydrates the fabric, the cell-free system would activate. If bacterial infection is present, AHL molecules could diffuse into the fabric, trigger transcription, and the system would produce antimicrobial peptides directly at the wound site.

What societal challenge or market need does this address?

Antibiotic resistance is one of the most pressing global health crises. A bandage that could autonomously detect and respond to infection without requiring systemic antibiotics would potentially reduce unnecessary antibiotic use, catch infections earlier than visual inspection, and could be especially valuable in low-resource or remote settings where medical monitoring is limited.

How do you address the limitations of cell-free reactions?

Rehydration as the activation trigger is actually an advantage in this design — the system would stay inert until wound fluid is present, avoiding premature activation. One-time use is not a significant limitation for a bandage, which would be replaced regularly anyway. Freeze-drying with trehalose as a lyoprotectant could extend shelf life sufficiently for practical storage and distribution. Reaction duration remains a challenge, but incorporating a sustained-release hydrogel layer could potentially maintain local humidity and extend activity over the critical early infection window.

Genes in Space Proposal

Background: What is the space biology challenge?

Astronauts on long-duration missions are exposed to chronic radiation that damages DNA and raises cancer risk. Current dosimeters measure physical exposure but can’t report on what’s actually happening at the cellular level. A freeze-dried cell-free biosensor could potentially offer a simple, equipment-light way to monitor biological DNA damage in real time — something especially valuable when medical resources are scarce.

Molecular or genetic target:

A fluorescent reporter (GFP) under control of the recA promoter, which is activated by the bacterial SOS DNA damage response.

How does your target relate to the challenge?

The recA promoter responds directly to DNA damage — the more damage, the stronger the activation. Using it to drive GFP expression in a cell-free system could provide a biological readout of radiation harm, rather than just a physical measurement of exposure.

Hypothesis or research goal:

I hypothesize that a freeze-dried cell-free system containing a recA-GFP construct could act as a simple biological radiation dosimeter. If radiation causes sufficient DNA damage in the sample, the recA promoter would activate and drive GFP expression, producing a fluorescent signal readable with the P51 viewer — no live cells or complex equipment required.

Experimental plan:

Freeze-dried BioBits reactions containing the recA-GFP construct would be rehydrated and exposed to varying doses of UV radiation as a proxy for space radiation. GFP output would be measured with the P51 viewer across a dose range. Controls would include shielded reactions (negative control) and a constitutive GFP construct to confirm the cell-free system is functioning. A clear dose-dependent fluorescence increase would support moving toward spaceflight validation.