Engineers have dreamed for decades of programming organisms to produce ethylene, a chemical known as "the king of petrochemicals" due to its importance in plastics. One promising route to this petrochemical is now becoming a reality, thanks to a photosynthetic bacterium that is genetically specialized to convert sunlight and carbon dioxide (CO2) into ethylene.
However, before the industry can begin stockpiling tanks of living green liquid, researchers must first overcome some metabolic barriers associated with ethylene production.
A multi-institutional research team led by the National Renewable Energy Laboratory (NREL) made significant strides toward understanding the photosynthetic enzyme pathway. The researchers report their discovery and demonstration that a specific gene can induce stability in bacteria that produce ethylene in a Nature Communications article titled "A guanidine-degrading enzyme controls the stability of ethylene-producing cyanobacteria."
"Until now, a major impediment to photosynthetic ethylene production has come from the organism itself—it produces a toxic byproduct alongside the ethylene," Jianping Yu, an NREL author on the paper, explained. "We now know that the toxic byproduct can be dealt with using a genetic technique as a result of this work."
Guanidine: An Undesirable Guest
The researchers' intended approach is simple: Take the ethylene-producing gene from a common plant pathogen (Pseudomonas Syringae, a bacterium that causes brown spots on leaves) and insert it into a cyanobacterium, which uses photosynthesis for energy.
If all goes well, the cyanobacteria will convert solar radiation and CO2 into ethylene more efficiently than any other biological pathway. Instead, the cyanobacteria died slowly; the researchers discovered that the introduced gene pathway also produces guanidine, a toxin that creates genetic instabilities in cyanobacteria.
"Our goal is to figure out what causes guanidine toxicity in this pathway and how cells can avoid it. To that end, we now have a fairly compelling strategy "Yu stated.
Guanidine disrupts pigment metabolism in cyanobacterial cells, which is an obviously undesirable byproduct given that the cells' purpose is to harvest light using their pigment. Fortunately, guanidine can be degraded by a cyanobacterium known as Synechocystis 6803.
The trick is to extract that genetic mechanism and reintroduce it into other cyanobacteria cells. In other words, introduce a second gene that stabilizes the first and results in unhindered ethylene production.
Genomic Stability for Enduring Ethylene Yields
Based on the gene's higher expression in the ethylene-producing strain and its sequence similarity to other known guanidine-related metabolic machinery, the researchers hypothesized that a specific gene in Synechocystis 6803 was involved in guanidine degradation.
When the researchers knocked the gene out of the cyanobacterium and observed the cells' decline when exposed to guanidine, their hypothesis was confirmed. The researchers then added the gene to another species to further validate the gene's role in guanidine degradation.
The researchers tested whether the gene conferred the same ability to degrade guanidine in the other cyanobacterium, Synechococcus 7942, another favourite species that scientists engineered. As in the first species, the modified cyanobacterium could metabolize the guanidine, preventing genetic problems and allowing for long-term ethylene production. The gene effectively neutralized guanidine in both organisms, converting the toxic chemical into innocuous urea and ammonia.
Opportunity for a Clean Chemical Alternative
Biologically produced ethylene is a win-win for clean energy because it recycles CO2 while also displacing fossil-based feedstocks on which industry currently relies. In comparison to other biological pathways that use plant biomass as a starting material, the method pursued in this work is directly fueled by the sun, making it potentially more energy-efficient.
The industrial application to decarbonize the chemical industry is enticing; there is still hope for producing PVC pipes for clean water and even Mars colonization. This research suggests that by removing certain biological barriers, it is possible to scale up bio-ethylene production.
Future research could lead to the development of even more efficient guanidine-degrading enzymes, possibly through the evolution of the same gene described in this study. For the time being, the team's work advances our understanding of guanidine metabolism in nature and demonstrates a functional approach for enhancing ethylene production.
(Source: Phys.org)
