Rapid Characterization of Genetic Parts with Cell-Free Systems
August 30th, 2021
Well-characterized genetic parts are necessary for the design of novel genetic circuits. Here we describe a cost-effective, high-throughput method for rapidly characterizing genetic parts. Our method reduces cost and time by combining cell-free lysates, linear DNA to avoid cloning, and acoustic liquid handling to increase throughput and reduce reaction volumes.
Our method allows for the rapid characterization of genetic parts using cell-free expression systems and PCR instead of cloning. The main advantage of this technique is what typically takes days to weeks in cells can be done in hours to days using this method. Our protocol includes steps to make your own cell-free expression systems, which can be tricky in a number of places and may require adjustments depending on use case.
When first starting out, we recommend starting with commercial kits. Begin by preparing the PCR master mix according to the instructions in the manuscript and store it on ice. Aliquot 30 or 40 microliters of the master mix into the determined number of PCR tubes and add 10 microliters of each five micromolar variable primers to appropriately-labeled PCR tubes.
Place the PCR tubes into the thermocycler and run the PCR program as mentioned in the text manuscript. Then, hold the reactions at four degrees Celsius. If the original template is plasma DNA, add one microliter of DpnI restriction enzyme to digest the original template and incubate the reactions at 37 degrees Celsius for one hour.
Analyze five microliters of each PCR product by separating them using a 1%agarose gel at 180 volts for 20 minutes. Check for the correct band size, which will vary with the chosen core sequence and the length of the parts added. Purify the linear templates using a commercial PCR purification kit.
If multiple bands were present by gel electrophoresis, excise the bands of interest from the gel and purify the DNA using a commercial gel extraction kit, as per the manufacturer's instructions. Quantify each DNA template using a spectrophotometer, assessing its quality by checking that the 260 to 280 nanometer ratio is approximately 1.8. Thaw all the components on ice and prepare a master mix by mixing all the components thoroughly with a pipette, as mentioned in the manuscript.
Pay careful attention to avoid precipitation, especially for the amino acid mixture. Keep the master mix on ice. Chill a 384-well plate on ice and distribute the master mix in nine-microliter aliquots into each well.
Thaw the represser protein on ice and distribute it into an acoustic liquid handling source plate, ensuring that the appropriate amount of dead volume required for the type of source plate used is included. Distribute the represser protein in one-microliter volumes into the appropriate destination wells via the liquid handler. Include a serial dilution of the purified reporter or an appropriate chemical standard on the plate to compare results with other studies and other labs.
Choose a range of concentrations appropriate for the reporter used in the expected expression range of the experiments. Prewarm the plate reader to 30 degrees Celsius. If possible, on the instrument, set a one degree Celsius vertical temperature gradient to limit condensation on the seal.
Set the plate reader to read at settings appropriate for the reporter used in the core sequence, without shaking steps. Seal the 384-well plate with an impermeable plastic seal to prevent evaporation. Place the 384-well plate on the plate holder and begin reading.
15 linear templates, varying only in the distance of the T7 promoter relative to the tetO sequence, were prepared by PCR, amplifying the superfolder green fluorescent protein reporter, with primers designed to add each promoter variant. Analysis of data from a total of 540 reactions for the entire set of T7 tetO combinations showed that the T7 RNA polymerase downregulates T7-driven expression equally, up through 13 base pairs downstream from the start of the T7 transcript. More consistent dispensing across the series of eight destination wells using the acoustic liquid handler was observed when a five-microliter transfer was divided into separate one-microliter dispenses.
Our protocol can be used to rapidly screen genetic components, whether for screening of new biosensors or building of genetic parts'libraries.
Overview
This article presents a high-throughput method for the rapid characterization of genetic parts using cell-free expression systems. By combining linear DNA and acoustic liquid handling, the method significantly reduces the time and cost associated with traditional cloning techniques.
Key Study Components
Area of Science
- Genetic engineering
- Biotechnology
- Cell-free systems
Background
- Characterization of genetic parts is essential for designing genetic circuits.
- Traditional methods can be time-consuming and expensive.
- Cell-free expression systems offer a promising alternative.
- Acoustic liquid handling enhances throughput and reduces reaction volumes.
Purpose of Study
- To develop a cost-effective method for characterizing genetic parts.
- To streamline the process of genetic circuit design.
- To enable rapid screening of genetic components.
Methods Used
- Preparation of PCR master mix and cell-free expression systems.
- Use of acoustic liquid handling for precise dispensing.
- Analysis of PCR products via gel electrophoresis.
- Quantification of DNA templates using spectrophotometry.
Main Results
- The method allows for characterization in hours to days instead of days to weeks.
- 540 reactions were analyzed, demonstrating consistent results across multiple trials.
- Effective use of acoustic liquid handling improved dispensing accuracy.
- The protocol is adaptable for various genetic components and biosensors.
Conclusions
- This method significantly accelerates the characterization of genetic parts.
- It provides a reliable framework for future genetic engineering projects.
- The approach can facilitate the development of new biosensors and genetic libraries.
Frequently Asked Questions
Executive Industry Relevance
Rapid characterization of genetic parts accelerates synthetic biology workflows by reducing time from days to weeks to hours to days, enabling faster iteration in genetic circuit design. This approach supports predictive confidence in part function and facilitates high-throughput screening for biosensor development and genetic library construction. The method enhances translational continuity by providing data applicable to both cell-free and cellular contexts.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of genetic parts such as promoters, operators, ribosome binding sites, insulators, and terminators for functional validation.
- Operational Value: Uses PCR-amplified linear DNA without cloning, reducing workflow complexity and turnaround time.
- Predictive Value: Results from cell-free screening can predict function in whole cells, supporting target confidence assessment.
Screening & Assay Development
- Scientific Value: Acoustic liquid handling and 384-well plates enable high-throughput, reproducible evaluation of genetic parts with precise reagent dispensing.
- Operational Value: Eliminates overnight culture and normalization steps required in cell-based assays, increasing throughput and reducing hands-on time.
- Assay Readiness: Generates quantitative reporter protein expression data suitable for assay standardization and cross-study comparison.
Translational & Preclinical Research
- Translational Continuity: Characterized parts can be directly applied in cell-free systems or used to inform cellular expression predictions.
- De-risking: Early identification of functional parts reduces uncertainty in downstream genetic circuit performance.
- Scalability: Supports rapid prototyping of biosensors and genetic parts libraries for therapeutic and diagnostic applications.
Pipeline & Workflow Integration
The method fits within the discovery continuum from genetic part screening to circuit assembly and functional validation, enabling rapid progression from hypothesis to prototype.
- Discovery Biology: Supports hypothesis testing of genetic regulatory elements through quantitative reporter output in a controlled cell-free environment.
- Screening: Delivers assay-ready, standardized measurements of part strength and compatibility via fluorescence-based reporter systems.
- Analytics: Provides quantitative, normalized expression data that enables comparison across part variants and experimental conditions.
- Translational Research: Outputs can inform cellular behavior predictions, bridging in vitro characterization with in vivo function.
- Enterprise Reuse: Protocol is adaptable across multiple genetic components and reusable for iterative library screening and optimization.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence in genetic part function and reduces mechanistic ambiguity in circuit design.
- Operational Value: Standardizes workflow through linear DNA preparation, acoustic dispensing, and sealed plate reading to minimize variability.
- Strategic Value: Accelerates go/no-go decisions in synthetic biology projects by enabling rapid evaluation of numerous part combinations.
- Portfolio Impact: Supports risk-adjusted prioritization of genetic constructs for further development in biosensor or therapeutic applications.
Implementation Considerations
- Requires expertise in molecular biology techniques including PCR, gel electrophoresis, and spectrophotometry.
- Dependent on access to thermocycler, plate reader, acoustic liquid handler, and 384-well plate infrastructure.
- Necessitates optimization of cell-free lysate conditions and reagent concentrations for specific reporter systems.
- Involves careful handling to prevent precipitation, particularly of amino acid mixtures in the master mix.
- Limited by the need for sequence-specific primer design and validation of linear template purity and integrity.
