Destruxins are cyclic hexadepsipeptides, ring-shaped molecules assembled from amino acids and hydroxy acids, that more than forty known analogs call home. They are the chemical weapons that Metarhizium fungi deploy against insect hosts, disrupting immune cells and muscle function, and they also mediate the fungus’s interactions with plants. Beyond agriculture, destruxins have attracted attention in medical research for antiviral, antimicrobial, anticancer, antiangiogenic, and immunosuppressive properties. Yet the environmental triggers that switch their biosynthesis on or off have remained murky, with reports in the literature sometimes contradicting one another.

The research team, led by Pamela Vrabl at the University of Innsbruck’s Institute of Microbiology, hypothesized that light might act not as a simple trigger but as a modulator whose effects depend on the nutritional state of the culture. This idea rests on a growing body of evidence from other fungi, including Aspergillus nidulans and Penicillium species, where the same wavelength can either boost or suppress secondary metabolite production depending on the medium. To test this in Metarhizium, the team grew the production strain M. brunneum BIPESCO 5 in 4.5-liter stirred-tank bioreactors under two radically different diets: a nutrient-rich Sabouraud medium with 4 percent glucose, and a defined minimal medium in which nitrogen was the limiting nutrient.

Each medium was subjected to five illumination regimes: complete darkness, red light centered at 660 nanometers, green light at 528 nanometers, and blue light at 455 nanometers, all delivered at an intensity of 1.4 milliwatts per square centimeter, plus ultraviolet-A irradiation at 340 and 375 nanometers at 1.1 milliwatts per square centimeter. LED panels were mounted in front of the glass bioreactor vessels and shrouded in light-tight fabric to prevent cross-contamination between treatments. Irradiance was verified with calibrated photodiode power sensors and spectrometers, ensuring that differences in output could not be attributed to sloppy light measurement.

Across 21 independent bioreactor batch cultivations, each run for 90 hours with triplicate sampling at ten time points, the fungus followed the classic growth pattern of filamentous fungi under carbon excess: a lag phase, exponential growth terminated by depletion of the first essential nutrient, a prolonged deceleration phase, and finally stationary phase. In the nutrient-rich medium, phosphate ran out first; in the minimal medium, ammonium did. Destruxin production began only after exponential growth had ended, consistent with the general principle that secondary metabolites accumulate once primary growth stalls.

The first major result concerned nutrition alone. Cultures grown under nitrogen limitation produced roughly twice as much destruxin A and B in absolute terms as those in the rich medium—up to 42.9 milligrams per liter of destruxin A and 22.7 milligrams per liter of destruxin B in darkness, compared with 19.1 and 8.6 milligrams per liter respectively in the rich medium. When normalized to fungal dry mass, the gap widened dramatically: nitrogen-limited cultures produced up to five times more destruxin A and nearly seven times more destruxin B per gram of biomass than their well-fed counterparts. This aligns with the long-standing observation that reduced nitrogen availability tends to elevate secondary metabolite levels in fungi, although the authors note that the regulatory role of nitrogen in destruxin biosynthesis remains incompletely understood and that published reports on the topic vary considerably.

The second and more striking result was that light did matter—but its direction flipped with the diet. In the nutrient-rich medium, shorter wavelengths enhanced production: cultures exposed to ultraviolet-A, blue, or green light produced significantly more destruxin A than those kept in darkness or under red light, and biomass-normalized destruxin B also rose significantly with shorter wavelengths. In the nitrogen-limited medium, the same short-wavelength treatments had the opposite effect, significantly reducing destruxin A and B levels relative to darkness and red light. Red light, by contrast, behaved much like darkness in both media. Destruxin E, the third analog measured, showed no significant light response in either medium, though its concentrations fluctuated wildly between replicate cultures—possibly because the molecule hydrolyzes into a related form called destruxin E-diol.

Perhaps the most telling detail lay in the ratios. Although light exposure swung the absolute concentrations of destruxins A and B up and down, the ratio between the two remained essentially constant within each medium—roughly 70:30 in the rich medium and 60:40 under nitrogen limitation—regardless of wavelength. Statistical analysis confirmed strong positive correlations between the two metabolites across all light treatments, with Pearson correlation coefficients reaching 0.94 in the minimal medium and 0.97 when data from both media were pooled. This pattern suggests that the biosynthetic pathway for destruxins A and B, whose genes are known to be clustered in the Metarhizium genome, is governed primarily by nutritional status, while light modulates the total flux running through it rather than rerouting it toward one product or the other.

The findings place destruxin regulation alongside a broader phenomenon in fungal physiology: nutrient-dependent light responses, in which the same photic signal produces different metabolic outcomes depending on carbon and nitrogen availability. Similar crosstalk has been documented for sterigmatocystin production in Aspergillus nidulans, where changing the carbon source could reverse whether light or darkness favored toxin synthesis, and for the antibiotic xanthoepocin in Penicillium ochrochloron. For Metarhizium, whose ecological repertoire spans insect pathogen, soil saprotroph, and plant-associated endophyte, such flexibility makes evolutionary sense: the fungus encounters everything from absolute darkness underground to green-light-dominated leaf surfaces and full sun, and its chemistry presumably adapts accordingly.

For biotechnologists, the practical implications are immediate. Destruxin yields in fermentation could potentially be increased by co-optimizing nutrient formulation and illumination regime rather than treating light as an afterthought, and shifting the product profile toward a preferred analog might become feasible by tuning the two variables in concert. The authors caution that mechanistic and scale-up studies will be needed before such strategies reach the production floor, and they emphasize that rigorous standardization of both cultivation and light conditions—wavelength and intensity alike—will be essential for untangling the molecular machinery underneath. What the study establishes beyond doubt is that destruxin biosynthesis is not a simple on/off switch but a finely tuned adaptive system, one that listens to both the menu and the light before deciding what to make.

Subject of Research: Nutrient-dependent effects of light wavelength on destruxin biosynthesis in the entomopathogenic fungus Metarhizium brunneum

Article Title: Nutrient-dependent influence of red, green, blue, and UV-A light on the production of destruxin A, B, and E in Metarhizium brunneum bioreactor batch cultures

Article References: Schobert, J., Erckert, S. A., Schinagl, C. W., Schöbel, H., Strasser, H., & Vrabl, P. (2026). Nutrient-dependent influence of red, green, blue, and UV-A light on the production of destruxin A, B, and E in Metarhizium brunneum bioreactor batch cultures. Frontiers in Photobiology, 4, Article 1753405. https://doi.org/10.3389/fphbi.2026.1753405

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1753405

Keywords: Metarhizium brunneum, destruxins, fungal photobiology, secondary metabolism, bioreactor cultivation, nitrogen limitation, light wavelength, entomopathogenic fungi, biological pest control, UV-A irradiation, biotechnology, fungal physiology

Protein-glutamine glutaminase, often abbreviated PGG, belongs to a family of enzymes that modify glutamine residues within proteins. When applied to plant proteins such as those from soy, pea or wheat, it can convert glutamine side chains into glutamic acid, changing the charge profile of the protein and, in turn, its solubility, emulsifying capacity and gel-forming behavior. That matters enormously for the fast-growing plant-based food sector, where manufacturers struggle to replicate the mouthfeel and stability that animal proteins deliver naturally. An enzyme that can be sprinkled into a formulation to improve functionality without chemical additives is an attractive proposition, but only if it can be manufactured at scale, consistently and affordably.

The Barcelona group, led by Clàudia Lliso-Pascual, Sergi Abad, Marc Carnicer and Antoni Planas, working between the Institut Químic de Sarrià of Universitat Ramon Llull and the biotechnology company CYGYC BIOCON, chose Bacillus subtilis as their production host for good reason. This Gram-positive bacterium is famous for secreting proteins directly into the culture medium, a trait exploited for decades in the industrial production of enzymes for detergents and food processing. Secretion simplifies purification dramatically: instead of cracking open cells and separating a soup of intracellular contaminants, manufacturers can simply collect the culture supernatant, in which the target protein is already enriched. Yet B. subtilis does not secrete every protein equally well, and finding the right combination of genetic controls is largely a matter of trial and error.

To bring order to that trial and error, the researchers constructed a modular secretion platform based on Golden Gate cloning, a DNA assembly method that allows genetic parts to be swapped in and out of a common backbone with standardized overhangs. In their architecture, a promoter, which controls how strongly the gene is transcribed, and a signal peptide, which directs the growing protein through the secretion machinery, could be combined systematically in a single construct design. This meant that dozens of promoter-signal peptide pairings could be screened under identical conditions, with the gene of interest held constant. Such standardization is the essence of modern synthetic biology: rather than redesigning each construct from scratch, engineers assemble variants like interchangeable modules and let the data decide which configuration wins.

The screening combined a reporter assay with direct measurement of enzyme activity, allowing the team to identify which combinations genuinely delivered functional protein to the medium rather than merely producing transcripts. The clear winner was a configuration pairing the Pgrac100 promoter with the amyQ signal peptide, a signal sequence derived from the alpha-amylase gene that has a long track record in Bacillus secretion systems. In the laboratory strain B. subtilis 168, this construct supported robust secretion of the PGG enzyme from Chryseobacterium proteolyticum, designated CpPGG. Intriguingly, the secreted protein emerged as an inactive proenzyme, and the host bacterium itself carried out the maturation step, processing the proenzyme into its active form through its own extracellular proteases.

That host-mediated activation initially looked like a convenient bonus, but it became a liability at scale. When the researchers moved their best construct into 3-liter batch bioreactors running on rich medium, the culture reached 2.9 units per milliliter of enzyme activity in the supernatant, a respectable starting point for a difficult-to-secrete enzyme. However, when they shifted to fed-batch cultivation in a chemically defined medium, a strategy used industrially to push cell densities and product titers higher, a new problem surfaced. The very proteases that B. subtilis uses to mature the proenzyme began to degrade the accumulated protein, and extracellular proteolysis emerged as a major limitation under high-cell-density conditions. In other words, the same biology that activated the enzyme also destroyed it.

The solution was conceptually elegant: decouple secretion from activation. The team transferred their construct into B. subtilis KO7-S, a protease-deficient strain that has been engineered to lack many of the extracellular degradative enzymes that plague protein production in this organism. The strain was kindly provided by Dr. Jordi Ferrando and Prof. Pere Picart of the University of Barcelona, whose expertise in Bacillus biology and transformation supported the work. In KO7-S, the non-processed proenzyme accumulated stably in the culture supernatant, protected from the proteolytic buzz-saw that had undermined production in strain 168. The trade-off was that the accumulated protein was inactive, but that turned out to be a manageable problem rather than a fatal one.

With the proenzyme safely stockpiled outside the cells, the researchers activated it on their own terms. They treated the supernatant with a controlled in vitro activation step using a food-grade neutral protease, an enzyme acceptable for use in food processing, to cleave the pro-peptide and unleash the catalytic activity. This controlled approach yielded 12.8 plus or minus 0.8 units per milliliter of supernatant, a more than fourfold improvement over the batch bioreactor result in the protease-rich host. Just as importantly, the decoupling strategy gives manufacturers precise control over when and how the enzyme becomes active, which simplifies quality control and avoids the unpredictable mixture of intact and degraded protein that plagued the earlier system.

The broader significance of the study lies less in any single number than in the engineering logic it demonstrates. Efficient PGG production, the authors conclude, requires coordinated control of three distinct processes: secretion, extracellular stability and proenzyme activation. Each of these can be optimized independently, and the modular platform makes that optimization systematic rather than ad hoc. The same Golden Gate architecture could, in principle, be applied to other industrially interesting enzymes that suffer from poor secretion or proteolytic degradation in Bacillus hosts, turning the platform into a general-purpose chassis for extracellular protein production. The work also highlights the value of food-grade solutions throughout the process, from the neutral protease used for activation to the generally recognized safety profile of B. subtilis itself.

For the plant-based food industry, the prospect of a reliable, scalable supply of protein-glutamine glutaminase could accelerate the development of next-generation meat and dairy alternatives with improved texture, solubility and nutritional performance. For bioprocess engineers, the study offers a textbook example of how modular cloning, host strain selection and bioreactor strategy must advance together: a brilliant genetic construct is worthless if the fermentation destroys its product, and a high-titer fermentation is worthless if the protein it yields is inactive. The Barcelona team’s platform, funded in part by an industrial doctorate grant from the Catalan government’s AGAUR agency, bridges those worlds, and its open-access publication means that other laboratories can immediately adopt and extend the modular design. As demand for functional plant proteins continues to climb, engineered microbial factories like this one are likely to play an increasingly central role in feeding the world’s appetite for sustainable protein.

Subject of Research: Modular secretion engineering in Bacillus subtilis for scalable production of protein-glutamine glutaminase

Article Title: A modular secretion platform in Bacillus subtilis enables scalable production of protein-glutamine glutaminase

Article References: Lliso-Pascual, C., Abad, S., Carnicer, M., & Planas, A. (2026). A modular secretion platform in Bacillus subtilis enables scalable production of protein-glutamine glutaminase. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14008-4

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14008-4

Keywords: Bacillus subtilis, protein-glutamine glutaminase, protein secretion, Golden Gate cloning, modular cloning, proenzyme activation, bioreactor cultivation, plant proteins, food-grade enzymes, protease-deficient strains, synthetic biology, industrial biotechnology