Engineered TnpB Variants Enable Efficient Editing of Plant and Human Cells

Research Background: Why We Need Smaller Genetic Scissors

Gene editing technologies are revolutionizing agriculture and medicine; however, traditional tools such as Cas9 are relatively large in size, making their efficient delivery via viral vectors challenging. TnpB is a class of RNA-guided endonucleases—considered the evolutionary ancestor of CRISPR-Cas12—and is regarded as an ideal next-generation editing tool due to its low molecular weight and compact structure. Nevertheless, the editing activity of naturally occurring TnpB is generally weak, which has limited its practical application.

Recently, the teams led by Jennifer A. Doudna and David F. Savage published a study in Nature Biotechnology. By employing deep mutational scanning, they systematically elucidated the sequence-function relationships of ISDra2 TnpB, successfully developing highly active engineered variants that provide a new paradigm for the design of compact genome editing tools.

Research Methods: Deep Mutational Scanning Maps the Functional Landscape

The research team utilized Deep Mutational Scanning (DMS) technology to establish a positive selection assay for TnpB-mediated DNA cleavage within a yeast system. This method quantitatively assesses the impact of each specific mutation on nuclease activity by monitoring the growth of yeast cells in an adenine-deficient culture medium.

The experimental design involved two independent plasmid libraries:

• reRNA Library: Encompassing all single-nucleotide substitutions, single/double-nucleotide deletions, and stem-loop structure substitution variants—totaling 576 distinct mutants.
• Protein Library: Covering all possible single-amino acid substitutions and stop codon variants—enabling the interrogation of 93% (7,611 out of 8,140) of all potential mutations.

Each variant was associated with approximately 30 unique barcodes. By calculating the relative enrichment—derived from the ratio of barcode abundance under selective versus non-selective conditions—the researchers achieved high-throughput functional assessment of the variants.

Figure 1. Design of DMS libraries and optimized in vivo selection for endonuclease activity in yeast. (Thornton, et al. 2026)

Unexpected Activation Hotspots Exist within the RNA Scaffold

The reRNA component accounts for approximately half of the molecular weight of the TnpB ribonucleoprotein complex, and its 116-nucleotide scaffold is critical for cleavage activity. The study revealed the following findings:

• Truncations and deletions within the pseudoknot structure resulted in a loss of activity, consistent with expectations.
• Stable tetraloop substitutions in the Stem 2 region were enriched more readily than those in the Stem 1 region; notably, known Trim2 truncation variants exhibited high levels of enrichment.
• An unexpected discovery was the presence of an activating mutation hotspot within the Stem 2 hinge region (rA-37 to rU-44), where single-nucleotide deletions significantly enhanced activity.

In EGFP knockout experiments conducted in HEK293T cells, hinge-region deletion variants demonstrated higher editing efficiency than the wild-type; conversely, while Trim2 variants were highly enriched in yeast, their efficiency declined in mammalian cells. These results suggest that Stem 2 plays a pivotal role in regulating the release of the RuvC domain, and that variant activity is differentially influenced by distinct host environments.

20% of Amino Acid Substitutions Can Enhance Protein Activity

The protein DMS data reveals a rich functional landscape:

• 7% of single amino acid substitutions result in at least a two-fold enrichment in activity.
• Activating mutations are abundant within the RuvC and WED domains, whereas mutations in the zinc-finger domain predominantly lead to a loss of activity.
• Truncation of the C-terminal tail after residue 376 does not affect activity, consistent with in vitro
• Near the nucleic acid-binding interface, substituting negatively charged residues with positively charged amino acids can enhance activity—a finding consistent with optimization strategies for Cas12 enzymes.

Of particular note are the following observations:

• Sites N4 and L172 within the WED domain show a preference for aromatic and small-volume amino acids, respectively; these preferences may enhance the stability of the heteroduplex via π-π stacking and hydrophobic interactions.
• At site E302 within the central channel, mutations that eliminate the negative charge result in a significant increase in activity, presumably by reducing electrostatic repulsion with the phosphate backbone of the target strand.
• Site P282, located at the boundary of the lid subdomain, favors hydrophobic amino acids; this preference may increase domain flexibility and accelerate the conformational transition triggered by heteroduplex sensing.

Figure 2. Activating mutations inform mechanistic insights and engineering. (Thornton, et al. 2026)

Construction and Validation of Engineered Variants

Based on the DMS data, the research team selected 33 highly enriched single-point mutations spanning 19 distinct sites and constructed a library of approximately 5,000 combinatorial variants using nicking mutagenesis. Following two rounds of yeast-based selection, five highly active combinatorial variants (designated eTnpBa through eTnpBe) were successfully identified. Testing in HEK293T cells revealed the following:

• All five variants exhibited higher indel frequencies at multiple endogenous loci compared to the wild-type.
• eTnpBd (R110K; P282V; E302Q) demonstrated the best performance, achieving indel frequencies ranging from 23% to 42%.
• eTnpBe (L172G; V192L; L222I; P282V; I304R) exhibited the lowest off-target activity (<2%), striking a balance between high efficiency and specificity.

Breakthrough Performance in Plant Genome Editing

The application of compact editing tools in plants has historically been constrained by delivery efficiency and cargo capacity. This study evaluated the editing capabilities of eTnpBc and eTnpBe (designated as TnpB-KYLI and TnpB-VGIRL, respectively) across various plant species:

• Nicotiana benthamiana: At the NbPDS1 locus, TnpB-KYLI achieved indel frequencies of 33–45%, representing a 4- to 40-fold improvement over the wild-type. At the NbWRKY40 locus, TnpB-VGIRL achieved 38% indel frequencies, nearly matching the performance of Cas9 (39%). These results surpass other compact editors such as ISYmu1 (<7%) and AsCas12f-HKRA (<9%).
• Rice: In regenerated plantlets derived from stably transformed callus, the two variants achieved editing efficiencies of 25.3% and 29.3%, respectively, at the OsHMBPP locus.
• Pepper: In this non-model crop, Agrobacterium-mediated infiltration with TnpB-VGIRL resulted in a 10% editing efficiency at the CaAGO2 locus, whereas the wild-type achieved less than 1%.

Off-target analysis indicated that the engineered variants maintained off-target activity levels comparable to—or even lower than—those of the wild-type in N. benthamiana.

Mechanistic Insights and Evolutionary Implications

This study elucidates the core elements governing the regulation of TnpB activity:

• The Stem 2 hinge region acts as a regulatory switch, where its conformational flexibility directly influences the activation of the RuvC domain.
• The protein surface harbors a widespread landscape of evolutionarily accessible activating mutations; notably, 20% of single-amino acid substitutions were found to enhance activity—a frequency far exceeding that predicted by conventional protein evolution models.
• This high frequency of activating mutations suggests that native TnpB, within its transposon context, may be subject to negative selective pressure, wherein its nuclease activity is constrained to limit deleterious effects on host fitness.

The study further revealed that activating mutations identified in ISDra2 can be successfully transferred to its homologs—ISYmu1 and ISAba30—despite their low sequence similarity, thereby demonstrating the universality of this activation mechanism across the TnpB family.

Application Prospects and Significance

The successful engineering of TnpB-KYLI and TnpB-VGIRL marks a pivotal moment, ushering compact gene-editing tools into a phase of high efficiency and practical utility. Its significance is underscored by the following aspects:

• Viral Vector Delivery: The inherent advantage of their compact size enables efficient delivery via viral vectors—such as AAVs—thereby resolving the cargo capacity limitations often associated with larger molecular editors.
• Crop Improvement: The demonstration of highly efficient gene editing in Nicotiana benthamiana, rice, and pepper provides novel, non-transgenic tools for precision breeding.
• Therapeutic Applications: The high activity and low off-target editing rates observed in mammalian cells open up promising avenues for in vivo gene therapy.

The research team had previously demonstrated that TnpB-KYLI, delivered via viral vectors, achieved heritable editing rates exceeding 50% in the progeny of N. benthamiana—a performance significantly superior to that of the wild-type enzyme. With further optimization of delivery methods and the expansion of the TAM recognition repertoire, engineered TnpB variants are poised to emerge as a pivotal platform technology within the field of gene editing.

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Reference

1. Thornton, B. W., et al. (2026). Engineered TnpB genome editors for plants and human cells identified by ribonucleoprotein mutational scanning. Nature Biotechnology, 1-9. DOI: 1038/s41587-026-03059-7.