The microbial workhorse at the center of the study is Methylorubrum extorquens PA1, a facultative methylotroph that naturally grows on methanol and can also assimilate ethanol. Methylotrophs have long fascinated metabolic engineers because they can consume cheap, non-food carbon sources that do not compete with agriculture. Methanol, in particular, can be generated from methane, carbon dioxide and biomass, making it an attractive platform molecule for industrial biotechnology. Ethanol, too, is produced at enormous scale from fermentation and is easy to store, transport and handle. If a robust microbial chassis could be engineered to funnel these two-carbon and one-carbon molecules into complex aromatics, the result would be a flexible carbon-recycling platform suited to a circular economy.

Yet turning that vision into reality has proven difficult, and the new study illustrates precisely why. When the researchers profiled the metabolism of M. extorquens PA1 growing on ethanol, they discovered a critical bottleneck: the bacterium was inefficiently converting acetate into acetyl-CoA, the universal metabolic currency that fuels biosynthesis. As a result, acetate accumulated outside the cells and growth on ethanol suffered. In essence, the microbe was letting a large fraction of the carbon it consumed leak out of its metabolic network before that carbon could be routed toward anything useful. Identifying this choke point was the first major insight of the work, and it framed every engineering decision that followed.

The team’s solution was to overexpress an endogenous enzyme called acetyl-CoA synthase, known as MeACS1, which directly activates acetate into acetyl-CoA. This single intervention suppressed the wasteful extracellular acetate accumulation and measurably improved the bacterium’s growth on ethanol. The result underscores a broader principle in metabolic engineering: before a production pathway can be installed, the native assimilation routes feeding it must be able to keep up with demand. In methylotrophs engineered for methanol utilization, the analogous challenge often lies in formaldehyde detoxification and the serine cycle; for ethanol, the study suggests that relieving the acetyl-CoA bottleneck is likely the critical step for efficient assimilation of this C2 feedstock.

With the entry point strengthened, the researchers turned to the synthetic part of the project: building a biosynthetic module that could deliver cinnamic acid. Cinnamic acid is produced in plants from the amino acid L-phenylalanine by phenylalanine ammonia-lyase enzymes, which catalyze the non-oxidative deamination that strips the amino group and releases the aromatic acid. The team imported the gene for one such enzyme, EncP from the marine bacterium Streptomyces maritimus, into M. extorquens PA1. Heterologous phenylalanine ammonia-lyases have been deployed in engineered microbes before, but never in this methylotrophic context, and the enzyme gave the bacterium a direct route from its own phenylalanine pool to the target product.

Supplying enough L-phenylalanine then became the next rate-limiting problem. Phenylalanine is synthesized through the shikimate pathway, a seven-step route that connects central carbon metabolism to the aromatic amino acids. Its entry substrates are phosphoenolpyruvate, drawn from glycolysis, and erythrose-4-phosphate, drawn from the pentose phosphate pathway. The researchers attacked both sides of the precursor question. They co-expressed two feedback-resistant enzymes from Escherichia coli: a mutant version of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase, the first and most heavily regulated enzyme of the shikimate pathway, and a fusion of chorismate mutase with prephenate dehydrogenase that pushes flux from chorismate toward phenylalanine. By using feedback-resistant variants, the engineers ensured that accumulating aromatic intermediates could not slam the brakes on the very pathway they were trying to boost.

On the erythrose-4-phosphate side, the team overexpressed the endogenous transketolase of M. extorquens, MeTkt, an enzyme that helps channel carbon through the sugar-phosphate rearrangements that generate this key precursor. Together, these layered interventions shunted more carbon from ethanol assimilation through the shikimate pathway and into L-phenylalanine, and from there through the bacterial ammonia-lyase into cinnamic acid. The resulting strain, designated PAc1/pTE102-CA3, produced 1.75 milligrams per liter of cinnamic acid from ethanol in simple test-tube cultures. That figure may sound modest by industrial standards, but it represents 2.6 times the yield the same engineered network could achieve from methanol, an important demonstration that ethanol is a superior feedstock for this particular chassis.

The team then moved from test tubes to controlled fed-batch cultivation, the standard workhorse format of industrial fermentation, in which ethanol was supplied incrementally to avoid overwhelming the cells. Under these conditions the engineered bacterium accumulated 52.34 milligrams per liter of cinnamic acid, roughly a thirty-fold improvement over the tube cultures, while maintaining stable growth and without notable acetate accumulation. That combination of productivity and physiological stability is significant: many engineered strains collapse under the metabolic burden of aromatic overproduction, or excrete organic acids that poison the culture. Here, the early repair of the acetyl-CoA bottleneck appears to have paid dividends, keeping the carbon balanced as the production pathway drew heavily on central metabolism.

Why does cinnamic acid matter? The compound and its derivatives sit at the base of a large family of plant phenylpropanoids, including p-coumaric acid, caffeic acid, ferulic acid, cinnamaldehyde and a suite of polymers and pharmaceutical intermediates. A microbial route from ethanol to cinnamic acid could be extended, in principle, by adding downstream enzymes to reach these higher-value molecules. The study therefore positions M. extorquens PA1 not merely as a curiosity of methylotrophic metabolism but as a credible chassis for ethanol-based aromatic biosynthesis more broadly. It also provides a foundation for engineering other C2-assimilating bacteria, an emerging class of platform organisms that can feed on acetate and ethanol generated from gas fermentation and electrochemical carbon capture.

The work, supported by the RIKEN Center for Sustainable Resource Science, RIKEN’s Special Postdoctoral Researcher Program and a JST ACT-X grant, remains at an early stage, and the authors are candid that further engineering will be needed to push titers toward commercial relevance. Even so, the study delivers a clear technical roadmap: fix the acetyl-CoA entry point, install a proven ammonia-lyase, deregulate and amplify the shikimate and phenylalanine branches, and reinforce erythrose-4-phosphate supply. Each step is modular, and each can be iterated as new tools for editing methylotroph genomes mature. As biorefineries search for routes that decouple chemical manufacturing from fossil carbon, this small soil bacterium, now taught to drink ethanol and exude an aromatic acid, offers a compelling proof of principle for the carbon-recycling economy of the future.

Subject of Research: Metabolic engineering of Methylorubrum extorquens PA1 for cinnamic acid production from ethanol

Article Title: Metabolic engineering of Methylorubrum extorquens PA1 for cinnamic acid production from ethanol

Article References: Fujiwara, R., Mitsui, R., Akai, G., Tanaka, Y., & Shirai, T. (2026). Metabolic engineering of Methylorubrum extorquens PA1 for cinnamic acid production from ethanol. Biotechnology for Biofuels and Bioproducts. https://doi.org/10.1186/s13068-026-02833-8

Image Credits: AI Generated

DOI: 10.1186/s13068-026-02833-8

Keywords: Methylorubrum extorquens, metabolic engineering, cinnamic acid, ethanol, methylotroph, shikimate pathway, acetyl-CoA synthase, phenylalanine ammonia-lyase, biorefinery, biotechnology, RIKEN, renewable chemicals