New Dual-Plasmid System Cracks the Solubility Problem in E. coli Protein Expression

Recombinant protein production in Escherichia coli has long been the workhorse of molecular biology, powering everything from laboratory reagents to industrial enzymes and biopharmaceutical precursors. Yet for all its simplicity, cost-effectiveness and scalability, the platform continues to frustrate researchers with a familiar trio of problems: leaky expression that toxifies cells before induction, codon bias that stalls translation of foreign genes, and misfolding that sends precious proteins into insoluble inclusion bodies. A new study published in Applied Microbiology and Biotechnology describes an elegant two-plasmid architecture that tackles all three shortcomings simultaneously, and the reported results are striking enough to draw attention across the synthetic biology and protein engineering communities.

The system, developed by Yin Chen, Ke Zheng and colleagues at institutions across Guangxi, China, pairs a purpose-built expression plasmid called pSE2 with an auxiliary plasmid designated pRARE2a-GKJE. The design addresses a fundamental tension in plasmid-based expression: researchers want high copy numbers when preparing DNA for cloning, but low copy numbers during protein expression, when runaway replication amplifies leaky transcription and burdens the cell. Conventional expression plasmids lock users into a single copy-number regime, forcing compromises that either slow cloning workflows or degrade expression control.

The trick lies in a regulatory interplay mediated by two well-characterized bacterial proteins. The auxiliary plasmid supplies Rop, a small RNA-binding protein that represses replication of ColE1-family origins by stabilizing the interaction between RNA I and RNA II primers, effectively suppressing pSE2 copy number whenever the two plasmids coexist in the same cell. At the same time, the auxiliary plasmid delivers the LacI repressor at elevated levels, tightening transcriptional control over the expression cassette and further silencing basal transcription before induction. In practice, this means pSE2 can be propagated alone at high copy for efficient plasmid preparation, then be tamed by the companion plasmid once the two are brought together for expression.

Beyond replication and transcription control, pRARE2a-GKJE carries the cargo that directly attacks the solubility problem: rare transfer RNAs that compensate for codon bias in human-optimized genes, and molecular chaperones, including the GroEL-GroES and DnaK-DnaJ-GrpE systems indicated by the GKJE designation, that shepherd nascent polypeptides into their correct folds. Because previous auxiliary plasmids sharing the p15A origin and chloramphenicol marker could not be combined with one another, laboratories studying genes suffering from both codon bias and folding difficulty had no way to deploy tRNA supplementation and chaperone co-expression together. The new system resolves this incompatibility by consolidating all four functions, Rop, LacI, rare tRNAs and chaperones, onto a single companion plasmid.

The performance figures reported for challenging human-codon-optimized targets illustrate the payoff. TurboID, a promiscuous biotin ligase widely used in proximity labeling, reached 97 percent soluble expression. SpCas9, the genome-editing nuclease that has transformed molecular biology yet remains notoriously prone to misfolding in bacterial cytoplasm, achieved 99 percent solubility. PE6d, a prime-editing-associated protein, reached 53 percent soluble expression. For laboratories that routinely spend weeks optimizing induction conditions, strains and lysis protocols to rescue a few percent of soluble material from inclusion bodies, these numbers represent a substantial reduction in the trial-and-error burden of difficult-protein expression.

The copy-number dynamics add a second layer of practical value. During cloning, pSE2 maintained the high DNA yields that make plasmid preparation and downstream molecular work fast and economical. Upon co-expression with pRARE2a-GKJE, the plasmid population dropped markedly, minimizing leaky expression of products that may be toxic, membrane-active or proteotoxic even at basal levels. This conditional behavior, high copy when alone and suppressed copy in the expression strain, effectively decouples the cloning and expression phases of a project that conventional single-plasmid designs force into a single compromise.

One further feature broadens the system’s appeal beyond protein production. The authors report that pSE2 enables direct eukaryotic functional validation without re-cloning, meaning a construct prepared for bacterial expression can be carried forward into eukaryotic testing without the traditional subcloning step. For gene-editing reagents, proximity-labeling enzymes and other tools whose value depends on functional screening in eukaryotic cells, this removes an entire workflow bottleneck and reduces the opportunities for sequence errors and cloning artifacts to creep in along the way.

The significance of the work lies less in any single engineering novelty than in the integration. Rop-mediated copy-number control, LacI-mediated transcriptional tightening, tRNA supplementation and chaperone co-expression have each been explored individually over decades of E. coli expression research. By assembling them on a compatible two-plasmid chassis, the Guangxi team has converted a collection of partial fixes into a coherent platform that behaves rationally across the full life cycle of a construct, from plasmid prep through induction to eukaryotic validation. The system offers what the authors describe as a versatile platform for both recombinant protein production and functional studies.

The research was supported by funding from the Scientific Research and Technology Development Program of Guangxi Zhuang Autonomous Region, the Natural Science Foundation of Guangxi Zhuang Autonomous Region, the National Natural Science Foundation of China, the Guangxi Qingmiao Talent Funding Project, the Guangxi Academy of Medical Sciences and the Guangxi Key Laboratory Operation Subsidy Project. Corresponding author Ke Zheng led the team alongside co-authors Yin Chen, Jialin Luo, Han Li, Zhuning Mo and Ben Huang. The article is published open access under a Creative Commons Attribution license, making the detailed protocols available to any laboratory seeking to adopt the system.

As demand grows for difficult-to-express proteins, from CRISPR effectors and base editors to engineered antibodies and proximity-labeling enzymes, tools that reliably deliver soluble, functional product from a cheap bacterial host carry broad relevance. If the reported solubility figures hold across a wider range of targets in other laboratories, the dual-plasmid system could become a standard fixture in expression workflows, shortening the path from gene sequence to functional protein for some of the most challenging molecules in modern biology.

Subject of Research: Development of a dual-plasmid system for soluble recombinant protein expression in Escherichia coli

Article Title: A dual-plasmid system for efficient soluble protein expression in Escherichia coli

Article References: Chen, Y., Luo, J., Li, H., Mo, Z., Huang, B., & Zheng, K. (2026). A dual-plasmid system for efficient soluble protein expression in Escherichia coli. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14039-x

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14039-x

Keywords: dual-plasmid system, Escherichia coli, recombinant protein expression, protein solubility, pSE2, pRARE2a-GKJE, molecular chaperones, rare tRNAs, leaky expression, codon bias, SpCas9, plasmid copy number