To make a protein with a cell-free expression kit, combine the kit’s required DNA or mRNA template with its supplied transcription-and-translation reaction, then incubate under the conditions in that kit’s manual. There is no universal recipe: template design, reaction ingredients, temperatures, and timing vary among wheat-germ, bacterial-lysate, and purified-component systems.
What a cell-free expression kit does
Cell-free protein synthesis (CFPS) makes protein outside living cells. A reaction supplies the machinery and reagents needed to transcribe a DNA template into RNA and translate RNA into protein, or to translate an mRNA template directly. Depending on the system, the template may be plasmid DNA, linear DNA, or mRNA. Some kits use a cell extract; others use purified components. NEB’s overview of cell-free protein expression describes these approaches and their differing applications.
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The kit manual—not another product’s protocol—determines which template, sequence elements, reagents, volumes, and incubation conditions to use. For example, a system may require a promoter and ribosome-binding site on the DNA, while an mRNA-based setup has different requirements.
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Before you start: match the kit to your template and target
- Template: Confirm whether the kit accepts plasmid DNA, linear DNA, mRNA, or a subset of these, and check the required promoter or other sequence features.
- System: Find out whether the reaction uses cell lysate or purified components. NEB describes PURExpress as a purified-component system and NEBExpress as an E. coli lysate-based system.
- Target and application: CFPS can be useful for rapid screening and protein engineering. Some systems can support work with toxic proteins, modified amino acids, or particular post-translational modifications, but those capabilities are not universal.
- Supplies and handling: Check what is included, what must be supplied separately, reaction scale, storage temperatures, extract freeze-thaw limits, and the assay you will use to detect or measure the protein.
- Controls: Use the kit’s recommended positive control for a first run when available. A control helps distinguish a setup problem from an issue with your target template.
Follow the kit’s workflow
A typical workflow is to prepare a suitable template, assemble the reaction as directed, incubate it, and test for the expected protein. Some products combine transcription and translation in one reaction; others separate the stages. Do not transfer reagent amounts or incubation settings between kits.
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Example: a wheat-germ kit with separate transcription and translation
The Sigma-Aldrich CFPS700 wheat-germ protocol illustrates a staged workflow:
- Prepare a DNA transcription template. Confirm that the construct has the sequence elements required for transcription and translation by this kit.
- Transcribe RNA. The protocol’s example uses T7 RNA polymerase at 37 °C for three hours, with up to six hours specified as an upper limit.
- Purify and check the mRNA. The protocol separates transcription from translation and calls for purification and confirmation of the resulting mRNA before using it.
- Set up translation. Combine wheat-germ extract, amino-acid mix, mRNA, and the other components specified by the manual. In its example 110 µL translation mixture, the protocol warns that adding more than 10 µL of wheat-germ extract may reduce yield.
- Incubate and assay. The example uses 16 °C overnight, for more than ten hours. Use the kit’s recommended method to detect or measure the protein.
Those temperatures, times, and volumes are specific to the cited CFPS700 protocol; they are not general settings for cell-free expression.
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Example: coupled bacterial-lysate systems
In a coupled system, transcription and translation can occur in the same reaction. Promega’s S30 T7 High-Yield Protein Expression System manual describes an E. coli extract containing T7 RNA polymerase and components needed for translation. The user supplies cloned DNA with the required T7 promoter and ribosome-binding site. Follow that product’s manual for the exact template requirements and assembly instructions.
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- Protect RNA and reactions from RNases. Use the RNase-free handling practices specified in the manual, especially when preparing or transferring mRNA.
- Store extract as directed. A CellFree Sciences wheat-germ kit manual, copyright July 2024, specifies storage at −80 °C and warns that repeated freeze-thawing can inactivate the extract. Follow the storage instructions for your own product.
- Start small. For an initial test, use the kit’s recommended scale and positive control rather than scaling up before confirming that the reaction works.
- Keep conditions product-specific. Lysate-based, purified-component, wheat-germ, and coupled systems are not interchangeable recipes.
If no protein is detected
Check the simplest failure points first: whether the template is intact and designed for the chosen system, whether reagents were stored and handled correctly, and whether the assay can detect the protein at the expected level. If the kit has separate transcription and translation stages, test them independently when the source of failure is unclear. The CellFree Sciences wheat-germ kit manual recommends stage-by-stage troubleshooting and RNase-free handling.
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For a first experiment, a kit-provided positive control can show whether the reaction is functional even if the target template or assay needs attention. A negative control can also help identify background signal; a published 2016 bacterial lysate protocol includes a negative-control recommendation, but your kit manual should guide the controls appropriate to your setup.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to interpret yield claims
Yield is not a reliable cross-kit promise: it depends on the system, target, template, reaction conditions, and measurement method. Treat a vendor’s figure as specific to the stated product and optimized template, not as an expectation for every protein. For example, the Cell-Free (Juice) E. coli kit page reports product-specific yields for optimized templates; those claims should not be generalized to other templates or kits.
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