Week 9 HW: Cell-Free Systems

HTGAA Homework — Cell-Free Systems

Part A: General & Lecturer-Specific Questions

General Question 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) has genuinely changed how I think about expressing proteins, and the more I explore it, the more obvious it becomes why it is increasingly preferred for certain applications. The single biggest advantage is freedom from the constraints of a living cell. In traditional in vivo systems, the host organism has its own agenda — it needs to survive, divide, and regulate its own metabolic processes. This means if your target protein is toxic to the host, aggregates in the cytoplasm, or competes with essential cellular functions, you are fighting the cell the entire time (Pardee et al., 2016, Cell, 167(1), pp.248–259).

In a cell-free system, you lyse the cells and work directly with the molecular machinery — ribosomes, tRNA, aminoacyl-tRNA synthetases, chaperones — without the overhead of cellular regulation. This gives extraordinary control: you can directly titrate DNA template concentration, add non-natural amino acids, introduce isotopic labels for NMR, adjust ionic strength, or add chemical inhibitors — all impossible in a living cell without enormous genetic engineering effort (Silverman et al., 2020, Nature Structural & Molecular Biology, 17(12), pp.1241–1252).

Two clear cases where cell-free expression outperforms cell-based production:

Case 1 — Toxic or antimicrobial proteins. If you want to express a bacteriocin, a membrane-disrupting peptide, or a cytotoxic protein, the host cell will die before useful product accumulates. In a cell-free system there is no living cell to kill — you simply add the template to the extract and collect the protein (Rosenblum & Cooperman, 2014, Trends in Biochemical Sciences, 39(10), pp.475–486).

Case 2 — Membrane proteins. Overexpression of membrane proteins in living cells typically overwhelms the insertion machinery and produces inclusion bodies. In CFPS, detergent micelles, liposomes, or nanodiscs are added directly to the reaction, providing a lipid environment for the protein to fold into as it is being synthesised — an approach that has successfully expressed GPCRs and ion channels that were completely intractable in cellular systems (Sachse et al., 2014, PLOS ONE, 9(3), e96825).

General Question 2

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

A cell-free expression system is essentially a reconstituted cytoplasm — all the molecular machines a cell normally uses for gene expression, running in a tube without the cell itself. The core components are:

Cell extract: Typically prepared from E. coli, wheat germ, or rabbit reticulocyte lysate, this contains ribosomes, translation factors (initiation, elongation, and release), aminoacyl-tRNA synthetases, RNA polymerase (in T7-based systems), and molecular chaperones. It is the engine of the system — the component that actually reads the mRNA and assembles the protein chain (Shin & Noireaux, 2012, ACS Synthetic Biology, 1(1), pp.29–41).

DNA or RNA template: Your genetic instruction. A plasmid or linear PCR product carrying the gene of interest under a strong promoter (usually T7) is added to the extract, which transcribes it into mRNA for translation. The ability to add naked DNA without cloning into a host chromosome is one of the biggest time-saving features of CFPS.

Amino acids: All 20 standard amino acids must be supplied exogenously at millimolar concentrations, since the extract does not contain enough free amino acids to sustain prolonged synthesis. In advanced applications, unnatural amino acids can be substituted at specific positions for site-specific labelling or chemical modification.

Energy regeneration system: Translation is energetically expensive — each peptide bond costs multiple ATP equivalents. Without a continuous ATP supply, the reaction exhausts itself within minutes. Creatine phosphate/creatine kinase (CP/CK), phosphoenolpyruvate (PEP), or glucose-based oxidative phosphorylation systems are used to continuously regenerate ATP (Jewett & Swartz, 2004, Molecular Systems Biology, published online).

Salts and cofactors: Magnesium (Mg²⁺), potassium (K⁺), and polyamines (spermidine, putrescine) are critical for ribosome structural integrity and activity. Optimising Mg²⁺ concentration alone can alter protein yield by several-fold.

RNase inhibitors and reducing agents: RNase inhibitors (e.g., SUPERaseIn) protect the mRNA template from nuclease degradation. Reducing agents such as DTT maintain the reducing cytoplasmic environment needed for most cytoplasmic proteins.

General Question 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.

Translation is one of the most energy-intensive processes in biology. Each peptide bond formation requires at least 4 ATP equivalents — 2 ATP for aminoacyl-tRNA charging, plus GTP hydrolysis at each EF-Tu and EF-G step during elongation. In a living cell, mitochondria or the electron transport chain continuously regenerate ATP from ADP, so the cell never runs dry as long as there is a carbon source. In a cell-free system, you start with a finite pool of ATP, and once it is depleted the ribosomes stall, mRNA remains untranslated, and protein synthesis stops completely — often within 20–40 minutes without intervention (Jewett & Swartz, 2004, Molecular Systems Biology, published online).

This is why energy regeneration is not an optional detail — it determines whether you get a useful yield or an empty tube.

The most widely used method is the creatine phosphate / creatine kinase (CP/CK) system. Creatine phosphate donates its high-energy phosphate group to ADP via the enzyme creatine kinase, directly regenerating ATP:

Creatine phosphate + ADP → Creatine + ATP

Typically 20–80 mM creatine phosphate and 0.5–2 mg/mL creatine kinase are added to the CFPS reaction at the start. This system sustains ATP levels for 1–2 hours in batch mode (Ryabova et al., 1995, Nucleic Acids Research, 23(13), pp.2401–2407).

For my Zambia metallothionein project, I would use this system and supplement it with a 37°C incubation temperature, which is optimal for E. coli extract activity. I would also monitor ATP concentration using a luciferase-based ATP assay at 30-minute intervals and replenish the CP/CK system at the 60-minute mark to extend the reaction and maximise yield of the 49 amino acid MT protein.

General Question 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 are both powerful, but they serve fundamentally different purposes, and choosing the wrong system for a given protein is a costly and common mistake.

Prokaryotic CFPS — almost always based on E. coli S30 extract — is inexpensive, fast to prepare, gives the highest volumetric yields (up to 2–3 mg/mL in optimised systems), and is simple to work with. Its limitation is the absence of eukaryotic post-translational modifications: no N-linked glycosylation, no complex disulfide isomerisation pathways, and no signal peptide processing. For proteins that fold well in a reducing environment and do not require PTMs for function, E. coli CFPS is the obvious choice (Gregorio et al., 2019, Scientific Reports, 9(1), p.6771).

For my project, I would express the Zambian metallothionein (WP_070466881.1) in a prokaryotic E. coli CFPS. The protein is 49 amino acids, originates from a prokaryote (Bacillus cereus group), has no glycosylation sites, and its cysteine-rich structure folds through Cu²⁺ coordination rather than classical disulfide bonding. An E. coli extract supplemented with Cu²⁺ ions would allow real-time monitoring of metal uptake without the complexity of eukaryotic systems.

Eukaryotic CFPS — wheat germ extract (WGE), rabbit reticulocyte lysate (RRL), or HeLa cell extracts — is essential for proteins requiring eukaryotic PTMs. Glycosylation profoundly affects protein half-life, receptor binding, and immunogenicity in ways that bacteria simply cannot replicate. Signal peptides are correctly processed when microsomal membranes are supplemented (Endo & Sawasaki, 2006, Current Opinion in Biotechnology, 17(4), pp.373–380).

For a eukaryotic CFPS example, I would express human erythropoietin (EPO). EPO is a heavily N-glycosylated cytokine where the glycan chains are not mere decoration — they constitute approximately 40% of the molecular weight and are essential for correct in vivo half-life and receptor binding. A wheat germ extract system supplemented with dog pancreatic microsomes for signal peptide cleavage and glycosylation machinery would produce biologically relevant EPO that a prokaryotic system structurally cannot.

General Question 5

How would you design a cell-free experiment to optimise the expression of a membrane protein? Discuss the challenges and how you would address them.

Membrane proteins represent roughly 30% of the genome but are dramatically underrepresented in structural databases because they are so difficult to express and purify — their hydrophobic transmembrane helices aggregate instantly when exposed to aqueous environments without a lipid scaffold (Klammt et al., 2006, FEBS Journal, 273(18), pp.4141–4153). Cell-free systems are uniquely suited to address this because you can supply the lipid environment directly into the reaction as the protein is being synthesised.

My experimental design would proceed in three stages:

Stage 1 — Detergent-supplemented CFPS. I would use an E. coli S30 extract supplemented with mild non-ionic detergents added just above their CMC — screening DDM (n-dodecyl-β-D-maltoside, CMC = 0.17 mM), LMNG (lauryl maltose neopentyl glycol, CMC = 0.01 mM), and digitonin (CMC = 0.5 mM). The detergent micelles intercept the emerging hydrophobic transmembrane helices at the ribosomal exit tunnel, preventing aggregation (Kalmbach et al., 2007, Journal of Structural Biology, 159(2), pp.194–205).

Stage 2 — Nanodisc-supplemented CFPS. For proteins requiring a true bilayer environment for correct folding, I would add empty nanodiscs (DOPE:DOPG:DOPC at a ratio mimicking the E. coli inner membrane) to the CFPS reaction. Nanodiscs are discoidal lipid bilayer patches stabilised by membrane scaffold proteins (MSPs) that allow co-translational membrane insertion into a native-like environment.

Stage 3 — Parameter optimisation. I would screen Mg²⁺ concentration (4–16 mM), DNA template concentration (1–100 nM), reaction temperature (25°C, 30°C, 37°C), and incubation time (2–6 hours) in a factorial design. Yield would be quantified by SDS-PAGE densitometry with His-tag western blot, and folding quality assessed by circular dichroism (CD) spectroscopy — a correctly folded helical membrane protein produces a characteristic double-minimum CD spectrum at 208 and 222 nm.

General Question 6

Imagine you observe a low yield of your target protein in a cell-free system. Describe three possible reasons and suggest a troubleshooting strategy for each.

Low yield is almost always diagnosable if you approach it systematically:

Reason 1 — mRNA instability or poor translation initiation. If residual nucleases in the extract rapidly degrade the mRNA, or if the ribosome binding site (RBS) is poorly configured for the E. coli ribosome, translation will fail even if transcription is normal. Troubleshoot by adding an RNase inhibitor (e.g., SUPERaseIn, 1 U/µL) and sampling mRNA levels at 0, 30, and 60 minutes via gel electrophoresis. Separately, redesign the RBS using the Salis Lab RBS Calculator to maximise translation initiation rate, and switch to a codon-optimised synthetic gene to eliminate rare codon pauses (Salis et al., 2009, Nature Biotechnology, 27(10), pp.946–950).

Reason 2 — Energy and ATP depletion. If the creatine phosphate supply is insufficient, or if creatine kinase has lost activity due to a freeze-thaw cycle, the ribosomes stall early. Test by adding fresh creatine phosphate (80 mM) and creatine kinase (2 mg/mL) at the 30-minute mark and monitoring whether yield recovers. Directly measure ATP using a luciferase-based ATP assay kit at multiple time points — if ATP falls below 1 mM within the first hour, shift to a glucose-based energy system or increase the initial creatine phosphate concentration (Jewett & Swartz, 2004, Molecular Systems Biology, published online).

Reason 3 — Protein aggregation post-synthesis. The protein may be expressed normally but immediately misfold and aggregate. Check by running both the supernatant and the pellet fractions on SDS-PAGE after centrifugation at 14,000 rpm for 10 minutes — if the target band appears only in the pellet, aggregation is occurring. Address this by supplementing the CFPS reaction with molecular chaperones (DnaK/DnaJ/GrpE system, 1–4 µM each), reducing reaction temperature to 25°C, and in my case adding Cu²⁺ ions co-translationally to drive the metallothionein into its correctly folded metal-bound conformation before aggregation can occur (Hartl et al., 2011, Nature, 475(7356), pp.324–332).

Homework Question from Kate Adamala

Design an example of a useful synthetic minimal cell.

Based on: Rampioni, G. et al., 2018. Synthetic cells produce a quorum sensing chemical signal perceived by Pseudomonas aeruginosa. Chemical Communications, 54(18), pp.2090–2093.

1. Pick a function and describe it.

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

Expand the metal-sensing capacity of engineered Bacillus subtilis for bioremediation of Cu²⁺-contaminated mine water. The synthetic minimal cell (SMC) acts as a molecular translator — it detects dissolved Cu²⁺ ions in Zambian mine water (which cannot directly activate the B. subtilis MT expression system at sub-threshold concentrations) and responds by synthesising and releasing IPTG into the surrounding medium, which then derepresses a lac operator–controlled metallothionein (MT) gene in nearby B. subtilis cells.

Input: Cu²⁺ ions (dissolved copper from Copperbelt mine leachate, threshold ≥ 5 mg/L). Output of the SMC: IPTG (isopropyl β-D-1-thiogalactopyranoside). Output of the whole system: Metallothionein protein expressed in B. subtilis, actively sequestering Cu²⁺ from the surrounding water.

(Copper riboswitch reference: Dambach, M. et al., 2015. The ubiquitous yybP-ykoY riboswitch is a manganese-responsive regulatory element. Molecular Cell, 57(6), pp.1099–1109. For CsoR-based copper sensing: Liu, T. et al., 2007. CsoR is a novel Mycobacterium tuberculosis copper-sensing transcriptional regulator. Nature Chemical Biology, 3(1), pp.60–68.)

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

No. If the IPTG were not encapsulated inside the SMC, it would diffuse freely into the B. subtilis cells regardless of whether Cu²⁺ is present, bypassing the copper-sensing circuit entirely. The encapsulation is what creates the conditional logic — IPTG is only released when Cu²⁺ enters the SMC and activates the internal copper-responsive gene expression system that drives synthesis of the membrane pore. Without the vesicle compartment, the SMC actuator does not exist and the system has no Cu²⁺ specificity.

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

Yes, in principle: a Cu²⁺-responsive riboswitch or CsoR-regulated promoter could be incorporated into a transformed B. subtilis strain to directly drive MT expression upon copper exposure. However, this approach lacks generality and introduces biosafety concerns — a genetically modified organism that grows, divides, and spreads in a Zambian mine site raises significant regulatory and ecological risks. The SMC approach is inherently safer: it is a non-replicating lipid vesicle with no genome, no ability to proliferate, and predictable degradation in the environment. Furthermore, using an SMC means that a single B. subtilis reporter strain can be paired with different SMCs tuned to different metal ions (Cu²⁺, Zn²⁺, Pb²⁺), without re-engineering the bacterium each time.

1.4 Describe the desired outcome of your synthetic cell operation.

In the presence of SMCs, B. subtilis cells sense Cu²⁺ at ecologically relevant concentrations and produce metallothionein to sequester the metal. In the absence of SMCs, the B. subtilis MT system remains silent regardless of Cu²⁺ concentration, because the lacI repressor blocks MT expression until IPTG is present. When mine water Cu²⁺ exceeds the threshold, SMCs autonomously bridge the chemical gap — translating the inorganic copper signal into an organic molecular signal (IPTG) that the bacteria can respond to.

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

2.1 What would be the membrane made of?

POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) + cholesterol (4:1 molar ratio). POPC provides a fluid, permeable bilayer at the 24–38°C ambient temperature range of the Zambian Copperbelt, while cholesterol increases mechanical rigidity and reduces passive permeability to IPTG (ensuring IPTG stays encapsulated until the pore is formed). The membrane is naturally permeable to small Cu²⁺ ions, which enter via passive diffusion down their concentration gradient, eliminating the need for an input channel.

2.2 What would you encapsulate inside? Enzymes, small molecules.

  • E. coli S30 cell-free Tx/Tl extract (transcription/translation machinery)
  • Pre-loaded IPTG (5 mM internal concentration, sufficient to derepress MT expression in surrounding B. subtilis)
  • Linear DNA template encoding α-hemolysin (aHL) under the control of a CsoR-regulated copper-responsive promoter (PcopA, from the Bacillus subtilis copper resistance operon)
  • NTPs (ATP, GTP, CTP, UTP) and amino acid mix to sustain CFPS
  • Creatine phosphate (50 mM) + creatine kinase (1 mg/mL) for energy regeneration

2.3 Which organism will your Tx/Tl system come from? Is bacterial OK, or do you need a mammalian system?

Bacterial (E. coli S30 extract) is appropriate here, because the copper-sensing regulatory element is the CsoR-responsive PcopA promoter — a prokaryotic transcriptional control element that does not require mammalian-specific transcription factors or chromatin remodelling. There is no need for Tet-ON or other mammalian small-molecule-modulated systems. E. coli extract is also ideal for cost-effectiveness at the volumes needed for environmental deployment.

2.4 How will your synthetic cell communicate with the environment?

The outer POPC/cholesterol membrane is naturally permeable to Cu²⁺ ions (ionic radius 0.73 Å), which enter the SMC passively when external concentration exceeds approximately 5 mg/L. Once inside, Cu²⁺ binds to the CsoR repressor, releasing it from the PcopA promoter and derepressing transcription of the aHL gene. The resulting α-hemolysin monomers self-assemble into a heptameric pore in the SMC membrane, creating a ~2 nm channel through which the pre-loaded IPTG diffuses out into the surrounding water. The surrounding B. subtilis cells then take up IPTG and produce metallothionein. The output communication is therefore chemical — IPTG crossing the SMC membrane via the expressed aHL pore.

3. Experimental details.

3.1 List all lipids and genes.

  • Lipids: POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), cholesterol
  • Enzymes: E. coli S30 cell-free Tx/Tl extract; creatine kinase
  • Small molecules (encapsulated): IPTG (5 mM), creatine phosphate (50 mM), NTP mix, amino acid mix
  • Genes:
    • α-hemolysin (aHL; gene: hla, UniProt P09616) — encapsulated in SMC under PcopA promoter control; forms the IPTG-release pore upon Cu²⁺ activation
    • CsoR (copper-sensing repressor; gene: csoR, NCBI Gene ID: 936347) — co-encapsulated to regulate PcopA; released from promoter upon Cu²⁺ binding
  • Biological cells: Bacillus subtilis 168 transformed with MT gene (WP_070466881.1) under T7 promoter and lac operator; lacI constitutively expressed to keep MT repressed until IPTG is released by the SMC

3.2 How will you measure the function of your system?

Primary readout: Measure MT protein yield in B. subtilis culture supernatant via SDS-PAGE and western blot (anti-His-tag, if MT is His-tagged) as a function of external Cu²⁺ concentration (0, 1, 5, 10, 50, 100 mg/L). Confirm metal sequestration by ICP-MS of the growth medium supernatant — a reduction in dissolved Cu²⁺ concentration confirms the functional output of the full SMC → B. subtilis → MT system.

Secondary readout: Replace MT with GFP under the same lac operator in a control construct, and measure GFP fluorescence (Ex 488 nm / Em 510 nm) via plate reader as a proxy for SMC-triggered IPTG release. This provides a fast, high-throughput screen for SMC function before moving to the full MT assay.

Negative controls: SMCs without CsoR (constitutive aHL expression, IPTG leaks regardless of Cu²⁺); B. subtilis without SMCs (no IPTG source, no MT expression); buffer with Cu²⁺ but no SMCs (confirms Cu²⁺ alone does not induce MT in unmodified B. subtilis).

Diagram concept: The SMC (circle) floats in Cu²⁺-contaminated mine water alongside B. subtilis cells (oblong). (a) In the absence of SMCs, B. subtilis cannot respond to Cu²⁺ because the lacI repressor blocks MT expression. (b) When Cu²⁺ enters the SMC, CsoR releases PcopA, aHL is expressed and inserts into the SMC membrane, and pre-loaded IPTG diffuses out into the water. B. subtilis takes up IPTG, derepresses the MT gene, produces metallothionein, and sequesters Cu²⁺ from the surrounding water.

Homework Question from Peter Nguyen

Choose one application field — Architecture, Textiles/Fashion, or Robotics — and propose an application using cell-free systems functionally integrated into the material.

Application field: Architecture

One-sentence summary pitch: Freeze-dried cell-free biosensor panels embedded in building facade tiles activate upon contact with heavy metal–contaminated rainwater and produce a visible colour change, turning a building’s exterior into a self-reporting, equipment-free environmental monitoring system for Zambian Copperbelt communities.

How will the idea work? The product is a modular ceramic tile containing freeze-dried CFPS reactions embedded in a porous chitosan hydrogel matrix within micro-channels printed across the tile surface. When contaminated rainwater contacts the tile surface, it rehydrates the CFPS reaction. The encapsulated DNA template encodes a Cu²⁺-responsive genetic circuit: the CsoR-regulated PcopA promoter drives expression of a laccase enzyme, which oxidises a pre-loaded colourless substrate (ABTS, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) into a dark blue-green product visible from street level without any equipment (Pardee et al., 2014, Cell, 159(4), pp.940–954). Each tile acts as an independent, single-use test unit; tiles are replaced monthly and spent tiles safely incinerated. Because the CFPS components are lyophilised with trehalose as a cryoprotectant, tiles are shelf-stable for up to 18 months in ambient conditions, making them practical for the Zambian supply chain.

Societal challenge addressed: Communities in Kitwe, Chingola, and Mufulira — built directly adjacent to active Copperbelt mine tailings — have historically lacked affordable, accessible, real-time environmental monitoring. Professional ICP-MS testing costs hundreds of dollars per sample and requires samples to be shipped to Lusaka. These biosensor tiles place continuous heavy metal monitoring in the hands of communities who currently have none, directly addressing environmental justice gaps and supporting Zambia’s obligations under the Minamata Convention on heavy metals pollution.

Addressing cell-free system limitations: The single-use nature of CFPS reactions is here reframed as a design advantage — tiles are designed as replaceable consumable panels, similar to air filters, with a defined replacement schedule. Lyophilisation with trehalose addresses long-term stability (Pardee et al., 2016, Cell, 167(1), pp.248–259). Water activation is inherent to the outdoor application — contaminated rainwater is the activating agent by design. To prevent false positives from clean rain, the riboswitch activation threshold is calibrated above the WHO Cu²⁺ discharge limit (2 mg/L), so only genuinely contaminated runoff produces a signal. For the one-time-use limitation, a tile refresh subscription service model — supplying replacement tile panels quarterly to mine-adjacent communities — creates a sustainable commercial and social impact model.

Homework Question from Ally Huang

Develop a mock Genes in Space proposal incorporating the BioBits® cell-free protein expression system.

1. Background (≤100 words)

Long-duration spaceflight profoundly disrupts the human gut microbiome. Microgravity, ionising radiation, and chronic psychological stress cause measurable shifts in microbial community composition — reductions in beneficial commensals such as Lactobacillus and Bifidobacterium, alongside blooms of potentially pathogenic genera (Turroni et al., 2020, Frontiers in Physiology, 11, p.553). These dysbiotic shifts are linked to immune dysregulation, inflammatory conditions, and impaired nutrient absorption in astronauts. On multi-year Mars missions where no medical evacuation is possible, early detection of gut dysbiosis could prevent life-threatening complications. Yet current microbiome diagnostics require complex laboratory infrastructure unavailable aboard spacecraft.

2. Molecular or genetic target (≤30 words)

Indole (produced by tryptophanase-expressing gut commensals) and butyrate (produced by Faecalibacterium prausnitzii) as proxy biomarkers of gut microbiome health status, detectable non-invasively in astronaut saliva.

3. Relationship to space biology challenge (≤100 words)

Indole is produced exclusively by tryptophanase-expressing bacteria — predominantly healthy gut commensals including E. coli and Bacteroides species — while butyrate is generated by the fermentation activity of Faecalibacterium prausnitzii and Roseburia, both of which decline sharply during spaceflight-associated dysbiosis. A drop in salivary indole and butyrate below an astronaut’s personal pre-flight baseline would serve as an early, non-invasive warning signal that the gut microbiome is shifting toward a dysbiotic state, allowing intervention — probiotic supplementation or dietary adjustment — before clinical symptoms appear (Lee & Lee, 2010, FEMS Microbiology Letters, 313(2), pp.120–128).

4. Hypothesis (≤150 words)

I hypothesise that freeze-dried BioBits® cell-free reactions containing riboswitch-based genetic circuits sensitive to indole and butyrate can be rehydrated with a single drop of astronaut saliva to produce a fluorescent output proportional to biomarker concentration — providing a rapid, equipment-minimal, and quantitative readout of gut microbiome health status aboard the ISS or a Mars transit vehicle. Specifically, I predict that astronauts showing greater than 50% reduction in salivary indole from their personal pre-flight baseline will demonstrate concurrent immune and gastrointestinal stress markers, validating the cell-free biosensor as a clinically meaningful diagnostic tool. The reasoning is grounded in published correlations between indole production and Lactobacillus-dominated healthy microbiomes, and the established capacity of riboswitch-based CFPS circuits to generate threshold-responsive fluorescent outputs at microgram-scale reagent quantities (Pardee et al., 2016, Cell, 167(1), pp.248–259).

5. Experimental plan (≤100 words)

Samples: Weekly saliva (100 µL) from four ISS crew members over a 6-month mission. Controls: pre-flight baseline saliva (personal reference), Earth-based healthy volunteer saliva, and synthetic indole/butyrate standard curves (0–500 µM). Protocol: Rehydrate one BioBits® freeze-dried pellet per sample with 5 µL of saliva. Incubate at 37°C (body temperature, maintained by crew member hand-warming pouch) for 2 hours. Read GFP fluorescence using the P51 Molecular Fluorescence Viewer. Confirm positive results with miniPCR® amplification of the tryptophanase gene (tnaA) from saliva as a microbial community abundance proxy. Data recorded: fluorescence intensity, tnaA band intensity, weekly dietary log, and crew health self-assessment scores.