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Engineering Thermothelomyces heterothallica for Cellobionate Production

This study engineered the thermophilic fungus *Thermothelomyces heterothallica* by disrupting specific catabolic genes via CRISPR-Cas9 to create strain TH11, which efficiently converts alkali-pretreated wheat straw into cellobionic acid with high yield and conversion rates without the need for external cellulases.

Original authors: Bakht Zada, Hamidreza Shapouri, Jiajie Wang, Sumit Sharma, Takao Kasuga, Noelia Valbuena, Ronen Tchelet, Mark Emalfarb, Zhiliang Fan

Published 2026-07-02

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CC BY 4.0

Original authors: Bakht Zada, Hamidreza Shapouri, Jiajie Wang, Sumit Sharma, Takao Kasuga, Noelia Valbuena, Ronen Tchelet, Mark Emalfarb, Zhiliang Fan

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a tiny, heat-loving factory worker named Thermothelomyces heterothallica. This isn't just any worker; it's a microscopic fungus that naturally loves to eat tough plant fibers (like wheat straw) and has a special talent for secreting its own "scissors" (enzymes) to cut them apart.

The scientists in this paper wanted to teach this fungus a new trick: instead of eating the plant fibers all the way down to simple sugar and using them for energy, they wanted the fungus to stop halfway and turn the plant parts into a valuable chemical called cellobionic acid (or CBA). Think of CBA as a "half-eaten" candy bar that is actually more useful to humans than the full bar.

Here is how they did it, broken down into simple steps:

1. The Problem: The Factory Worker Eats Too Fast

Normally, when this fungus eats plant fiber, it breaks it down into a sugar called cellobiose, and then immediately chops that up even further into glucose to fuel its own growth. It's like a worker who receives a package, opens it, and immediately eats the contents before anyone else can use them. The scientists wanted to stop the fungus from eating the "middle step" (cellobiose) so it would pile up and turn into the desired product (CBA) instead.

2. The Solution: A Genetic "Lockdown"

To fix this, the scientists used a high-tech editing tool (CRISPR-Cas9) to perform a series of genetic "surgery" operations on the fungus. They removed specific genes that acted like the fungus's digestive tools.

Step 1: They cut out the genes for eight different "scissors" (beta-glucosidases) that usually chop cellobiose into glucose. This stopped the fungus from breaking the middle step down further.
Step 2: They removed a gene for a machine called cellobiose phosphorylase, which usually eats the final product (CBA) if it's made. This ensured that once CBA was made, it stayed there.
Step 3: They cut out two genes that acted like doorways (transporters) which allowed the fungus to suck CBA back inside to eat it. By blocking these doors, the CBA was forced to stay outside in the liquid.

The result was a super-fungus named TH11.

3. The Process: Turning Straw into Gold

The scientists fed this new TH11 strain some alkali-pretreated wheat straw (a cheap, abundant waste product).

The Natural Talent: Because TH11 is a natural plant-eater, it didn't need any extra enzymes added to the mix. It used its own internal "scissors" to break the tough straw into cellobiose.
The New Trick: Because of the genetic changes, the fungus couldn't eat the cellobiose. Instead, it used a different enzyme (cellobiose dehydrogenase) to gently oxidize the cellobiose, turning it into cellobionic acid.

4. Tuning the Factory Conditions

The scientists realized that for the factory to run at peak efficiency, they needed to tweak the environment:

Temperature: The fungus loves heat. While it worked at 37°C, it worked best at 43°C. At this hotter temperature, the "scissors" moved faster, and the conversion happened much quicker.
pH Balance: Making CBA makes the liquid acidic (like adding lemon juice). The scientists added a citrate buffer (a pH stabilizer) to keep the environment from getting too sour, which helped the fungus keep working hard.

5. The Results

Under these perfect conditions (43°C with a pH buffer), the TH11 strain was incredibly efficient:

Speed: It produced a high amount of CBA in just 5 days.
Yield: It converted about 88% of the plant fiber into the product.
Efficiency: Of the plant fiber that was actually eaten, 96% was successfully turned into CBA.

Why This Matters (According to the Paper)

The paper highlights that this is a major improvement over previous methods. Earlier attempts used a different fungus (Neurospora crassa) that took 8 days to produce less product. The new TH11 strain is faster, works at higher temperatures (which reduces the risk of unwanted bacteria growing), and, most importantly, does not require expensive external enzymes to be added. It does all the work itself, turning cheap wheat straw directly into a valuable chemical.

In short, the scientists took a natural plant-eater, gave it a genetic "diet plan" to stop it from overeating, and tuned its environment to make it a highly efficient machine for turning agricultural waste into a useful chemical.

Technical Summary: Engineering Thermothelomyces heterothallica for Cellobionate Production

Problem Statement
The microbial production of cellobionic acid (CBA), a value-added sugar acid with applications in food, pharmaceutical, and chemical industries, is currently hindered by the high cost of feedstocks and the reliance on exogenous enzymes. Previous strategies utilizing Neurospora crassa demonstrated the feasibility of direct CBA production from lignocellulosic biomass but were limited by the host's relatively low cellulase production capacity and slower reaction kinetics. Furthermore, existing processes often require external cellulase supplementation, increasing operational costs. There is a need for a robust, thermophilic fungal host capable of high-level native enzyme secretion to facilitate the direct, enzyme-free bioconversion of lignocellulosic biomass into CBA.

Methodology
The study employed a metabolic engineering strategy using a Ribonucleoprotein (RNP)-based CRISPR-Cas9 system to engineer the thermophilic filamentous fungus Thermothelomyces heterothallica (formerly Myceliophthora thermophila). The engineering approach focused on redirecting carbon flux from cellulose degradation toward CBA accumulation while preventing product re-consumption.

The specific genetic modifications included:

Disruption of Catabolic Pathways: Sequential deletion of genes involved in cellobiose and CBA catabolism. This included the removal of eight $\beta$ -glucosidase genes (bgl) to prevent cellobiose hydrolysis into glucose, one cellobiose/cellobionate phosphorylase gene (cbp) to block CBA degradation, and two putative cellobionate transporter genes (cbt1 and cbt2) to prevent CBA uptake.
Strain Construction: Starting from the parental strain TH0, a series of mutants (TH3, TH6, TH9) were generated through successive rounds of gene deletion. The final strain, TH11, contained deletions in eleven genes total.
Process Optimization: Fermentation conditions were optimized regarding pH and temperature. The study utilized alkali-pretreated wheat straw (2% NaOH) as the substrate. Buffering with 50 mM citrate (pH 6.0) and varying temperatures (37°C, 40°C, 43°C) were tested to maximize yield and conversion rates.
Analysis: Substrate consumption, product titers, and enzyme activities (Cellobiohydrolase [CBH] and Cellobiose Dehydrogenase [CDH]) were monitored over 5 days. Cellulose conversion and CBA yield were calculated based on residual solids and supernatant analysis.

Key Results

Genetic Validation: Deletion of the phosphorylase gene in strain TH3 successfully abolished CBA utilization. However, CBA accumulation was unstable in intermediate strains (TH6, TH9) due to residual consumption. The final strain, TH11, which included the deletion of the two cbt genes, successfully prevented CBA re-consumption, allowing for continuous accumulation.
Substrate Utilization: Unlike the N. crassa heptuple $\beta$ -glucosidase mutant which struggled with cellobiose utilization, the T. heterothallica mutants efficiently utilized cellobiose. This was attributed to the host's native cre-1 mutation, which alleviates carbon catabolite repression, allowing CDH expression even in the presence of cellobiose.
Production Performance: Under optimized conditions (43°C, 50 mM citrate buffer, pH 6.0), strain TH11 produced 64 mM (approx. 12.8 g/L) of CBA from pretreated wheat straw within 5 days.
Conversion Efficiency: The process achieved an 88.0% cellulose conversion and a 96.2% yield of CBA from the consumed cellulose. Xylan conversion reached 95.6%.
Kinetics: The thermophilic strain demonstrated faster production kinetics compared to the previously engineered N. crassa HL10 strain (which required 8 days to reach 57 mM). TH11 reached a higher titer (64 mM) in a shorter timeframe (5 days).
Enzyme Activity: The strain naturally secreted high levels of CBH and CDH. CBH activity increased progressively, reaching 135 U/L, while CDH activity peaked at 441 U/L on day 3 before declining.

Significance and Claims
The paper claims that the engineered T. heterothallica strain TH11 provides a promising platform for the direct bioconversion of lignocellulosic biomass into value-added aldonic acids without the addition of external enzymes. The significance of this work lies in:

Process Efficiency: The use of a thermophilic host operating at 43°C accelerates reaction kinetics and reduces contamination risks compared to mesophilic systems.
Economic Viability: The ability of T. heterothallica to secrete high levels of native extracellular cellulases (>100 g/L capacity in industrial contexts) eliminates the major cost barrier associated with enzyme cocktails in lignocellulosic biorefineries.
Metabolic Engineering Success: The study validates that sequential disruption of catabolic genes, including transporters, is critical for stabilizing product titers in fungal hosts.

The authors note that while the conversion efficiency is high, the transient accumulation of cellobiose suggests the process is currently oxidation-limited. They suggest that future improvements could be achieved through enhanced oxygen supply in bioreactors and further engineering of oxidative metabolism, but do not propose specific new experimental designs beyond these general directions.