Genetic sequence allows photosynthetic organisms to stably produce ethylene

Cyanobacteria liquid culture is grown in Jianping Yu’s lab at NREL to advance sustainable ethylene production. Credit: Dennis Schroeder, NREL

For decades, engineers have dreamed of programming organisms to sustainably produce ethylene, a chemical dubbed “the king of petrochemicals” for its importance in plastics. Now, a hopeful path to this petrochemical is getting closer to reality, via a photosynthetic bacterium genetically specialized to transform sunlight and carbon dioxide (CO2) to ethylene. But before industry can fill reservoirs with living green liquid, researchers must first overcome some metabolic barriers around ethylene production.

An interagency research team led by the National Renewable Energy Laboratory (NREL) has made significant progress in deciphering the photosynthetic enzyme pathway. In one Nature Communication article titled “A guanidine-degrading enzyme controls the stability of ethylene-producing cyanobacteria”, the researchers report their discovery and demonstration that a certain gene can induce the stability of bacteria that produce ethylene. Their discovery is a welcome breakthrough, as past attempts to employ this ethylene pathway had led to genetic instability in the bacteria.

“So far, a major obstacle to photosynthetic ethylene production has come from the organism itself – it creates a toxic byproduct alongside ethylene,” said Jianping Yu, author of NREL on the item. “With this work, we now know that the toxic byproduct can be treated using a genetic technique.”

Guanidine: an unwanted guest in solar-powered ethylene production

The approach envisaged by the researchers is simple: hijack the gene to produce ethylene from a common plant pathogen (Pseudomonas Syringae, a bacterium that causes brown spots on the leaves) and introduce this gene into a cyanobacterium, which uses photosynthesis to produce energy. If everything is working correctly, the cyanobacteria will then convert solar radiation and CO2 in ethylene; indeed, more effectively than any other biological pathway. But instead, the cyanobacteria slowly died; the researchers showed that the introduced genetic pathway also produces guanidine, a toxin that creates genetic instabilities in cyanobacteria.

“Our goal is to understand the source of guanidine toxicity in this pathway and how cells can counteract it. To that end, we now have a pretty compelling approach,” Yu said.

Guanidine causes a disorder of pigment metabolism in cyanobacterial cells – an obviously bad by-product when the cells’ purpose is to use its pigment to harvest light. Luckily, one particular cyanobacterium favored by scientists, Synechocystis 6803, can degrade guanidine. So the trick is to capture this genetic mechanism and reinsert it into other cyanobacterial cells. In other words, introduce another gene that stabilizes the first one and leads to unobstructed ethylene production.

Genomic stability for sustainable ethylene yields

The researchers hypothesized that a specific Synechocystis 6803 gene was involved in guanidine degradation, based on the gene’s higher expression in the ethylene-producing strain and its sequence similarity to other known metabolic machinery related to guanidine. This supposition was confirmed when the researchers knocked out the cyanobacterium gene and observed the cells decline when exposed to guanidine. To further validate the gene’s role in guanidine degradation, the researchers then added the gene to another species.

In the other cyanobacterium, Synechococcus 7942, another favorite species created by the scientists, the research team assessed whether the gene conferred the same ability to degrade guanidine. Indeed, just like in the first species, the modified cyanobacterium could metabolize guanidine, thus preventing genetic problems and allowing a persistent production of ethylene. For both organisms, the gene effectively neutralized guanidine, converting the toxic chemical into harmless urea and ammonia.

Opportunity for a clean chemical alternative

Biologically produced ethylene is a double benefit for clean energy: it recycles CO2 and displaces the fossil raw materials that industry currently depends on. Compared to other biological pathways, which use plant biomass as feedstock, the method pursued in this work is powered directly by the sun, which makes it potentially more energy-friendly.

The industrial application to decarbonize the chemical industry is attractive; hope persists to produce PVC pipes for drinking water, and even in the colonization of Mars. This work shows a possibility of increasing the production of bioethylene by removing certain biological barriers. Future research could create even more efficient guanidine-degrading enzymes, possibly by evolution of the same gene described in this study. So far, the team’s work advances knowledge of guanidine metabolism in nature and demonstrates a functional approach to enhancing ethylene production.

Produce ethylene from food waste without greenhouse gas emissions

More information:
Bo Wang et al, A guanidine-degrading enzyme controls the genomic stability of ethylene-producing cyanobacteria, Nature Communication (2021). DOI: 10.1038/s41467-021-25369-x

Provided by National Renewable Energy Laboratory

Quote: Genetic Sequence Allows Photosynthetic Organisms to Stably Produce Ethylene (March 4, 2022) Retrieved March 4, 2022 from ethylene.html

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