Enhanced production of cis-13,16-docosadienoic acid in Yarrowia lipolytica via systematic metabolic engineering

Deep within the microscopic world of single-celled fungi, nature has built intricate factories capable of assembling complex molecules from simple ingredients like sugar. Among these molecules are fatty acids, the building blocks of oils and fats that play vital roles in both human health and the natural world. Some of these fatty acids, known as polyunsaturated fats, are particularly valuable because the human body cannot make them on its own; we must obtain them from our diet. One such molecule, a twenty-two-carbon chain called cis-13,16-docosadienoic acid, has recently captured the attention of scientists. Research suggests this specific fat possesses powerful properties, including the ability to slow the growth of certain cancer cells and fight inflammation. Until now, obtaining this substance has been a difficult task. It occurs naturally only in the seeds of a rare winter plant, which is slow to grow and hard to cultivate in large quantities. Chemical methods to create it in a lab are equally challenging, involving complicated steps and high costs. This has left researchers searching for a better way to produce it, turning their eyes toward the yeast Yarrowia lipolytica, a robust microorganism known for its ability to accumulate oils.

The challenge lies in convincing this yeast to make a molecule it does not naturally produce. The yeast is already quite good at making a precursor called linoleic acid, but it lacks the specific tools to stretch that precursor into the longer, more complex chain required for the final product. To solve this, a team of researchers at Hunan Agricultural University and other institutions treated the yeast like a biological workshop, systematically adding and adjusting the genetic tools needed to complete the job. They began by ensuring the workshop had an abundant supply of the starting material. By introducing genes from other organisms that act as specialized converters, they successfully increased the amount of linoleic acid inside the yeast cells. This step was crucial, as a lack of raw material often bottlenecks the entire production line.

With the supply of raw material secured, the team turned their attention to the assembly line itself. Converting the precursor into the final product requires a two-step stretching process. The researchers tested different versions of the enzymes responsible for this stretching, much like trying different tools to see which fits the job best. They found that one specific enzyme combination worked significantly better than others, successfully extending the fatty acid chain. However, the process was not yet efficient enough. The yeast was producing the intermediate step of the chain but struggled to finish the job. To fix this, the scientists tried a technique called enzyme fusion, where they physically linked two of the necessary enzymes together, hoping this would make the transfer of materials between them smoother. Surprisingly, this approach did not work as expected. The linked enzymes seemed to get in each other's way, and the production of the final product did not improve. This result taught the team that simply sticking tools together does not always make them work better; sometimes, the arrangement matters more than the connection.

Undeterred, the researchers shifted their strategy to focus on the power of the instructions driving these enzymes. They increased the number of copies of the genes responsible for the stretching process and tested different genetic switches, known as promoters, to control how strongly these genes were turned on. In a counterintuitive finding, they discovered that a weaker switch actually produced better results than a stronger one. This suggests that in the complex environment of a living cell, too much activity in one area can sometimes disrupt the balance of the whole system. By carefully tuning the expression of the genes, they created a strain of yeast that could convert the precursor into the desired fatty acid much more effectively than before.

The team also explored a different angle: preventing the yeast from breaking down what it had made. They removed a gene responsible for a cellular cleanup process that naturally degrades fatty acids, hoping this would allow the valuable product to accumulate. While this change did cause the yeast to store more oil overall, it had a disastrous side effect: the specific fatty acid they were trying to make disappeared entirely. The disruption of the cleanup system seemed to confuse the cell's metabolism so severely that it could no longer synthesize the target molecule. This ruled out that particular strategy, highlighting the delicate balance required in engineering living organisms.

Through this process of trial, error, and refinement, the researchers successfully engineered a strain of yeast that produced the highest amount of this valuable fatty acid ever recorded in a laboratory setting without using complex industrial fermentation equipment. In simple shake-flask cultures, the engineered yeast reached a concentration of 5.34 milligrams per liter, with the target molecule making up 0.47 percent of the total fat in the cells. This achievement demonstrates that by systematically understanding and adjusting the metabolic pathways of a microorganism, scientists can coax it into producing complex, high-value substances that were previously difficult to obtain. The work lays a foundation for future efforts to scale up this production, potentially offering a sustainable and accessible source of a molecule with significant promise for medicine and nutrition.