Cell-Free Systems

Cell-free protein synthesis (CFPS) eliminates the cell membrane as a barrier between the researcher and the transcription/translation machinery, which unlocks four categories of advantage over traditional in vivo methods:

• Direct access to the reaction. All components are accessible and modifiable in real time. Researchers can add, remove, or titrate individual molecules — polymerases, ribosomes, tRNAs, cofactors — without waiting for genetic transformations or growth cycles.
• Total molecular control. In PURE-system configurations, every molecule in the reaction is known and defined. This eliminates the biological noise inherent in whole-cell expression, where the host continuously produces its own proteome in competition with the target protein.
• Non-natural amino acid incorporation. CFPS reactions can be supplemented with orthogonal tRNA/aminoacyl-tRNA synthetase pairs that insert non-canonical amino acids at amber stop codons — a modification incompatible with living cells that cannot sustain suppressed stop codons across their entire proteome.
• No cellular viability constraint. The system does not need to keep a cell alive, so toxic products, destabilizing membrane proteins, and lethal genetic programs can all be expressed without selecting against the production host.

Two cases where CFPS clearly outperforms cell production:

• Toxic protein production. Proteins that kill or arrest host cells — bacteriocins, lytic phage proteins, cytotoxic cancer therapeutics — cannot accumulate in living cultures because selection pressure immediately eliminates high producers. A CFPS reaction has no cellular viability to protect: the lytic peptide Microcin J25 (the anti-coliform effector in Füzi Poiesis Strain A) could be produced and assayed in a CFPS reaction without the self-immunity proteins mcjC and mcjD that are required to protect the living E. coli host.
• Low-resource or field deployment. CFPS reactions can be lyophilized (freeze-dried) and stored at room temperature for months, then reactivated by adding water and template DNA. This enables on-demand biosynthesis without cold chain infrastructure — relevant for remote monitoring of Lake Budi water quality, where a freeze-dried CFPS biosensor loaded with a phosphate-responsive genetic circuit could be rehydrated in the field to produce a colorimetric or fluorescent readout of SRP concentration without laboratory access.

• Template DNA (plasmid or linear fragment). The instruction set. Contains the gene of interest under the control of a promoter compatible with the expression system (T7 for most E. coli CFPS kits). Provides the sequence that RNA polymerase will transcribe into mRNA.
• RNA polymerase. The transcription engine. Reads the DNA template and synthesizes the corresponding mRNA. In E. coli-based systems, T7 RNA polymerase is typically added exogenously because of its high processivity and orthogonality to host sigma factors.
• Ribosomes. The translation machines. Read the mRNA codon-by-codon and catalyze peptide bond formation between successive amino acids. In crude cell-free extracts, ribosomes are the most abundant and rate-limiting component — their concentration directly determines protein yield.
• Transfer RNAs (tRNAs) and aminoacyl-tRNA synthetases (aaRS). The decoders. Each tRNA carries a specific amino acid and recognizes the corresponding codon; aaRS enzymes charge tRNAs with the correct amino acids. Both are present in crude extracts; in PURE systems they are added as purified components with defined stoichiometry.
• Amino acids. The building blocks. The 20 canonical amino acids (or non-natural analogs) that ribosomes assemble into the polypeptide chain according to the mRNA sequence.
• Nucleotides (NTPs). The RNA building blocks consumed during transcription (ATP, GTP, CTP, UTP). Limiting NTP concentrations are a common cause of low yield in extended reactions.
• Energy regeneration system. ATP is the thermodynamic fuel of both transcription and translation, and is rapidly consumed. A phosphocreatine / creatine kinase system (or PEP / pyruvate kinase) regenerates ATP from ADP continuously, extending the productive lifetime of the reaction from minutes to hours.
• Buffer, salts, and cofactors. Mg²⁺ (typically as magnesium glutamate or magnesium acetate) is essential for ribosome structure and polymerase activity. K⁺ (potassium glutamate) maintains ionic strength. HEPES or Tris buffer stabilizes pH. NAD, CoA, and spermidine are added in some formulations to support cofactor-dependent enzymatic steps.

Unlike a living cell, which continuously regenerates ATP through glycolysis, oxidative phosphorylation, and the electron transport chain, a CFPS reaction begins with a fixed ATP pool that is consumed irreversibly with each phosphodiester bond formed during transcription and each peptide bond formed during translation. Without regeneration, the reaction exhausts its ATP supply within 20–40 minutes, arresting both transcription and translation long before the template DNA is depleted — the rate-limiting step shifts from polymerase or ribosome activity to thermodynamic fuel availability.

The most widely used regeneration method for E. coli-based CFPS is the phosphocreatine / creatine kinase (PCr/CK) system: creatine kinase catalyzes the transfer of the high-energy phosphate group from phosphocreatine to ADP, regenerating ATP continuously as long as PCr remains in the reaction. This system is simple to add, does not require membrane-bound enzymes, and sustains ATP levels for 4–6 hours in typical batch reactions.

An alternative with higher yield for extended reactions is the glucose-6-phosphate / glucose-6-phosphate dehydrogenase system, which couples NADPH regeneration to ATP synthesis, or the maltose / maltose phosphorylase system used in some commercial kits. For Füzi Poiesis CFPS validation in Aim 2 (testing the AND-gate circuit in a cell-free context before bacterial transformation), the PCr/CK system would be appropriate because of its compatibility with the E. coli-based myTXTL extract and its well-characterized kinetics in the 30°C reaction temperature range.

Prokaryotic system — protein choice: Microcin J25 precursor (McjA + processing enzymes McjB/McjC). The Microcin J25 lasso peptide is produced by E. coli from a four-gene operon (mcjABCD). Its post-translational lasso topology is installed by the bacterial enzymes McjB (macrolactamization) and McjC (further modification) — enzymes that are themselves of bacterial origin and function in a prokaryotic biochemical environment. An E. coli CFPS reaction can co-express all four genes simultaneously, allowing the lasso modification to occur in vitro and producing functional antimicrobial peptide without the self-toxicity problem that prevents accumulation in living E. coli hosts not protected by the mcjD immunity gene. This is directly relevant to Füzi Poiesis: CFPS validation of the mcjABCD cassette would confirm functional antimicrobial activity before committing to bacterial transformation in Aim 2.

Eukaryotic system — protein choice: Human alkaline phosphatase (ALPL, tissue-nonspecific isoform). While Füzi Poiesis uses bacterial PhoA, the human ALPL isoform is heavily N-glycosylated at five sites — glycosylation that is required for its correct folding, thermostability, and full catalytic activity. Expression in an E. coli CFPS system would yield aglycosylated, misfolded, largely inactive protein. A wheat germ or rabbit reticulocyte extract supplemented with canine pancreatic microsomal membranes provides the N-glycosyltransferases and the ER-equivalent membrane environment needed to produce correctly folded, glycosylated ALPL — relevant for studies comparing bacterial PhoA activity to the human enzyme under Lake Budi physicochemical conditions.

Membrane proteins are the most challenging targets for CFPS because they are intrinsically hydrophobic, aggregate in aqueous solution, and require a lipid bilayer or detergent environment to fold correctly. The standard CFPS reaction tube is an aqueous environment — the default state for a membrane protein expressed without a membrane is a misfolded insoluble pellet.

Design strategy for Füzi Poiesis context — MscL (mechanosensitive channel for the synthetic cell design):

• Detergent-supplemented CFPS. Add a mild non-denaturing detergent (n-dodecyl-β-D-maltoside, DDM, at 0.1–0.5% w/v) to the reaction to capture nascent MscL as it emerges from the ribosome, forming detergent micelles around the transmembrane helices and preventing aggregation. After the reaction, detergent can be exchanged for lipid during reconstitution into proteoliposomes.
• Liposome-supplemented CFPS. Pre-form POPC/POPG liposomes at the same composition as the synthetic cell membrane and add them to the CFPS reaction. Newly synthesized MscL inserts co-translationally into the liposome bilayer, bypassing the aggregation problem entirely. This approach directly produces proteoliposomes ready for vesicle fusion without a separate reconstitution step.
• Nanodisc-based capture. Nanodiscs (membrane scaffold protein belts around a lipid bilayer patch) can be added to the CFPS reaction as capture vehicles for MscL, producing water-soluble MscL-nanodisc complexes that retain channel function and can be characterized by single-channel electrophysiology.

Measurement: Channel function is validated by encapsulating MscL-proteoliposomes in a hypotonic solution and measuring calcein or sulforhodamine B release (fluorescence dequenching) upon osmotic stress — the channel opens mechanically and releases the dye. For the synthetic cell application, successful MscL expression and insertion would be confirmed by osmolarity-responsive vesicle swelling assayed by dynamic light scattering.

• Reason 1: Limiting Mg²⁺ or K⁺ concentration. Ribosome assembly and RNA polymerase activity are acutely sensitive to divalent cation concentration. Too little Mg²⁺ (below ~5 mM) destabilizes ribosome tertiary structure; too much (above ~15 mM) chelates the ATP energy supply and inhibits translation. Troubleshooting: perform a Mg²⁺/K⁺ grid titration across 2–20 mM Mg²⁺ and 50–200 mM K⁺ in 96-well format, measuring GFP fluorescence as a proxy for expression yield. Identify the optimum condition before applying to the target protein.
• Reason 2: mRNA secondary structure at the RBS or start codon. Stable hairpin structures in the 5′ UTR of the mRNA can sequester the Shine-Dalgarno sequence or the AUG start codon, blocking ribosome binding and dramatically reducing translation initiation. This is especially common when T7 RNAP produces transcripts without the optimized 5′ leader sequences used in vivo. Troubleshooting: use RNAfold to predict secondary structure of the first 50 nt of the transcript. If the RBS or AUG is buried in a helix (ΔG below −5 kcal/mol), redesign the 5′ UTR using a synthetic, structure-free leader sequence. For Füzi Poiesis phoA_opt, the RNAfold MFE of −22.60 kcal/mol for the RBS-CDS junction was validated as ribosome-accessible before synthesis.
• Reason 3: ATP depletion before reaction completion. In reactions without energy regeneration, or with insufficient phosphocreatine concentration, ATP is exhausted before all template DNA has been transcribed. Translation arrests abruptly, producing truncated proteins that are invisible on SDS-PAGE but account for the consumption of all building blocks. Troubleshooting: add a luciferase reporter to a parallel reaction and monitor luminescence over time — a plateau in luminescence signal followed by decay indicates ATP exhaustion. Increase phosphocreatine concentration from the standard 20 mM to 30–40 mM, or switch to a secondary energy regeneration system (glucose-6-phosphate) for reactions longer than 4 hours.

The synthetic cell replicates the logic of the entire Füzi Poiesis bacterial consortium in a single lipid vesicle compartment — a miniature sequential logic processor for conditional environmental bioremediation.

Function: Execute a multi-layer AND logic operation that activates a bioremediation module only when specific environmental criteria are simultaneously met.

• Layer 1 (Safety veto): Inputs X₁ (H₂S / toxicity) and X₂ (salinity / dissolved oxygen). If H₂S is present above threshold, the system is blocked regardless of other inputs — the PcstR promoter drives Csy4 endoribonuclease expression, which cleaves the downstream AND-gate mRNA. This prevents activation in anoxic conditions where the organism itself would be stressed.
• Layer 2 (Processing gate): If Layer 1 approves (low H₂S, appropriate salinity confirms open-water deployment), the PompC promoter drives traI AHL synthase expression, generating the Z₂ intercellular signal (3OC8-HSL).
• Layer 3 (Execution): Upon Z₂ reaching the executor vesicle, LuxR-AHL activates Plux, driving expression of PhoA (Y₁, phosphorus remediation), Enterocin A (Y₂, antimicrobial), and GFP (Y₃, reporter).

Inputs: X₁ = dissolved H₂S (passive diffusion through membrane), X₂ = osmolarity/salinity (MscL mechanosensitive channel), X₃ = soluble reactive phosphorus (PitA transporter). Outputs: Y₁ = alkaline phosphatase (PhoA), Y₂ = Enterocin A bacteriocin, Y₃ = GFP fluorescence.

Not autonomously in the lake environment. An unencapsulated CFPS reaction can execute the genetic circuits if supplied with DNA and nutrients in a controlled laboratory setting, but two critical properties are lost without a membrane boundary:

• Protection and concentration. The TX-TL machinery is fragile and would dilute instantly in a 3.8 km² lake, being degraded by environmental nucleases and proteases within minutes. Encapsulation creates a protected microenvironment that concentrates all components above the functional threshold for transcription and translation.
• Selective communication. The multilayer logic requires that H₂S can enter (passive), salinity can be sensed (via MscL mechanosensing), phosphate can enter (via PitA), and AHL can exit (passive) — while the TX-TL machinery remains contained and protected. Without a membrane, all of these signals would equilibrate with the bulk environment simultaneously, collapsing the sequential logic to a simple mixing reaction with no gating.

Yes — and this is exactly what Füzi Poiesis implements. The three-strain bacterial consortium (E. coli K-12 MG1655 as demonstration chassis, Halomonas elongata as the Aim 2 chassis) performs the same multilayer logic through genetically modified living cells coupled by cross-auxotrophic dependency. The advantage of living cells over synthetic cells is robustness and self-replication: the consortium can maintain itself, repair damaged components, and adapt to environmental variation through normal cellular physiology. The advantage of the synthetic cell design is that it requires no genetic modification of any organism and poses no horizontal gene transfer risk — a meaningful distinction for deployment in a lake that is a Lafkenche cultural and food sovereignty site.

The synthetic cell operates as a molecular guardian for Lake Budi's water column. Deployed in the Bokashi del Budi matrix at the littoral zone, it activates bioremediation only when conditions confirm open-water deployment (appropriate salinity) and absence of acute toxicity (low H₂S), with phosphorus excess as the trigger signal. The observable outcomes are:

• Reduction in soluble reactive phosphorus concentration in the surrounding water through PhoA-mediated hydrolysis of organic phosphate esters.
• Decrease in fecal coliform density through Enterocin A antimicrobial activity against Enterobacteriaceae.
• Green fluorescence emission from GFP, providing a non-destructive, field-observable confirmation that the full AND-gate logic executed successfully — conditions were met and the bioremediation program activated.

2.1 Membrane composition: Lipid bilayer of POPC (primary structural phospholipid, fluid bilayer at 15–25°C matching Lake Budi's littoral temperature range), POPG (net negative surface charge for colloidal stability and bacterial membrane mimicry), and cholesterol at 30–35 mol% (mechanical stability, reduced passive permeability to charged molecules). This composition allows passive diffusion of small uncharged molecules (H₂S, O₂, CO₂, AHL) while restricting ions (Na⁺, Pi²⁻) that require specific channels.

2.2 Encapsulated contents: E. coli CFPS extract (myTXTL — crude enriched extract containing RNA polymerases, ribosomes, tRNAs, amino acids, and ATP regeneration enzymes); plasmid DNA encoding all three processing layers; HEPES buffer pH 7.4; magnesium glutamate (10 mM); potassium glutamate (130 mM); phosphocreatine (25 mM) + creatine kinase (5 U/mL) as energy regeneration system.

2.3 Tx/Tl system organism: E. coli — optimal because all sensors (PcstR, PompC, LuxR/Plux quorum sensing), logic elements (Csy4 RNA endoribonuclease), and effectors (PhoA, traI AHL synthase) are of bacterial origin and function at maximum efficiency in a prokaryotic expression environment. Mammalian systems would be required only if small-molecule-modulated promoters like Tet-ON were needed, which they are not here.

2.4 Environmental communication: H₂S enters by passive diffusion through the bilayer (small, uncharged). Salinity/osmolarity is sensed by MscL (mechanosensitive high-conductance channel from E. coli) expressed from the encapsulated DNA and inserted into the vesicle membrane — the channel opens under osmotic stress, allowing ion equilibration that changes the internal ionic milieu sensed by PompC. Phosphate (Pi) enters via PitA (low-affinity phosphate transporter) expressed and membrane-inserted from the encapsulated DNA. AHL (Z₂ signal) exits by passive diffusion. PhoA and Enterocin A are secreted via Listeriolysin O (LLO) pores expressed upon Plux activation — LLO assembles 30–50 nm arciform lesions that allow release of globular proteins (PhoA ~6 nm, Enterocin A ~5 nm) too large for alpha-hemolysin heptamers (2 nm inner diameter).

Three orthogonal measurements confirm successful operation of all three layers:

• Layer 3 output (GFP, Y₃): Fluorescence intensity at 488/507 nm measured by flow cytometry (individual vesicle resolution) or plate reader (bulk). GFP signal above background in the (H₂S=LOW, Salinity=BUDI, Pi=HIGH) condition and below background in all OFF conditions confirms full AND-gate logic execution.
• Y₁ output (PhoA activity): Add pNPP (para-nitrophenyl phosphate) to the external medium after vesicle incubation. PhoA released through LLO pores hydrolyzes pNPP to para-nitrophenol (yellow, absorbance at 405 nm). Quantitative colorimetric assay with no equipment beyond a spectrophotometer.
• Y₂ output (Enterocin A antimicrobial): Co-incubate vesicles with a Listeria innocua reporter strain (natural target of Enterocin A). Zone of inhibition or growth arrest in co-culture confirms functional bacteriocin release. Alternatively, measure L. innocua OD₆₀₀ over 24 hours with and without activated synthetic cells.

How it works: Panels are fabricated from a porous biopolymer matrix (cellulose acetate or chitosan composite) loaded with lyophilized CFPS extract containing two genetic circuits: a riboswitch or TOEHOLD switch sensitive to a coliform-specific RNA sequence (a conserved 16S rRNA fragment from Enterobacteriaceae), and a reporter circuit driving β-galactosidase (LacZ) expression. When ambient humidity rehydrates the panel — from rain, condensation, or direct water contact — the CFPS reaction activates. If fecal coliform RNA is present in the rehydrating water, the toehold switch unfolds, ribosomes translate LacZ, and the enzyme converts a pre-loaded chromogenic substrate (CPRG, chlorophenol red β-galactoside) from red to yellow — a color change visible to the naked eye within 2–4 hours. Panels that contact uncontaminated water remain red.

Societal challenge addressed: The monitoring of water quality in rural coastal communities — including the Lake Budi basin — relies on laboratory sample collection and processing that takes 24–72 hours and requires cold chain and trained personnel. In communities where 64% of residents identify as Mapuche-Lafkenche and where artisanal fishing depends on daily water quality, a passive architectural sensor that reports contamination status visually to anyone who can see the wall of a community building represents a meaningful reduction in the infrastructure barrier to environmental justice.

Addressing CFPS limitations: Lyophilization stability is addressed by encapsulating the CFPS reaction in trehalose glass at 100 mM — trehalose forms a glassy matrix at low humidity that protects ribosome structure and DNA integrity for 12+ months at room temperature (demonstrated by Pardee et al., 2016). The one-time-use limitation is addressed architecturally: panels are designed as modular tiles (20×20 cm) that can be replaced individually after activation, like indicator paper. The water activation requirement becomes a feature rather than a limitation: the panel only activates when directly contacted by liquid water, creating a dose-responsive geographic map of contamination across the panel surface.

Long-duration spaceflight (lunar Gateway, Mars transit) requires reliable potable water recycling from crew waste streams. Current water quality verification relies on culture-based coliform testing (24–48 h) or electrochemical sensors that drift over time and require calibration standards. In a Mars transit scenario with 20-minute signal latency, a contamination event identified by culture-based methods represents 24–48 hours of crew exposure to potentially unsafe water. A rapid, reagent-independent, instrument-light biological detection system is needed that can be operated without microbiology laboratory infrastructure and validated under microgravity conditions.

The target is a conserved 30-nucleotide sequence within the V2 hypervariable region of 16S rRNA from Enterobacteriaceae — the family that includes E. coli, Salmonella, and Klebsiella, the primary fecal coliform indicators in water safety monitoring. This sequence is present in all Enterobacteriaceae but absent in the dominant non-pathogenic microbiome species found in ISS water systems (Sphingomonas, Methylobacterium), providing the specificity required to distinguish contamination from background.

The 16S rRNA is present at ~10,000 copies per bacterial cell — far higher than any protein-coding mRNA — making it detectable at the single-cell level without PCR amplification. This is critical for space applications where the miniPCR thermal cycler adds weight, power consumption, and operational complexity. A toehold switch that directly detects 16S rRNA from lysed cells bypasses amplification entirely: add water sample to BioBits reaction, lyse cells by heat (65°C, 5 min, achievable with the miniPCR heating block), and the toehold switch unfolds in the presence of target RNA, allowing ribosome access and LacZ translation. Color change from CPRG substrate is read visually with the P51 fluorescence viewer (no electricity required beyond a UV LED).

The central hypothesis is that the kinetics of toehold switch folding and CFPS-mediated LacZ translation are not significantly altered by microgravity, because both processes are governed by molecular diffusion and thermal energy — neither of which changes under weightlessness at the relevant reaction length scales (nanometers to micrometers). The research goal is to establish whether the detection threshold (minimum colony-forming units per mL of water needed to produce a visible color change within 4 hours) observed in 1g laboratory conditions is preserved in microgravity on the ISS, and whether the specificity against non-target ISS water microbiome species is maintained. This would validate freeze-dried CFPS toehold switches as a Standard Operating Procedure for water quality monitoring on long-duration missions without culture-based methods.

• Sample 1: BioBits reaction rehydrated with ISS potable water (uncontaminated baseline) — negative control.
• Sample 2: BioBits reaction rehydrated with ISS potable water spiked with heat-killed E. coli K-12 at 1, 10, 100, and 1000 CFU/mL equivalents — dose-response series.
• Sample 3: BioBits reaction rehydrated with ISS potable water spiked with Sphingomonas paucimobilis (predominant ISS water microbiome species, non-coliform) at 10,000 CFU/mL — specificity control.
• Sample 4: Identical reactions performed simultaneously in 1g on Earth by a ground control team — the paired comparison that tests the microgravity hypothesis.
• Measurement: Colorimetric readout (CPRG absorbance shift red→yellow) photographed with ISS camera and analyzed by ImageJ color quantification. Fluorescence confirmation with P51 viewer (LacZ generates fluorescent resorufin product). Time to visible color change recorded for each dose. Data transmitted to ground for comparison with 1g controls.