Researchers at UC Berkeley have come up with a more affordable way to produce QS-21, an important component in vaccines, using yeast instead of traditional, environmentally harmful methods.
A complicated process in yeast now enables the inexpensive production of QS-21, which was previously extracted from tree bark at high cost.
Vaccines play a crucial role in saving lives, especially during the recent pandemic. One often overlooked component is a substance that prepares the immune system to better defend the body against infection, a vital part of vaccines like the Novavax COVID-19 vaccine.
These substances, known as adjuvants, are added in small amounts but have a significant protective effect, especially in young children and older individuals.
One of the most powerful adjuvants comes from the Chilean soap bark plant, but its production cost is extremely high, reaching several hundred million dollars per kilogram.
A Remarkable Advancement in Synthetic Biology
Scientists at the University of California, Berkeley and Lawrence Berkeley National Laboratory (Berkeley Lab) have harnessed synthetic biology to produce QS-21, a crucial component of the soap bark, using yeast. This method is not only cost-effective but also more sustainable, avoiding the need for harsh chemicals used in plant extraction.
Although the amount of QS-21 produced from the yeast-based process is still limited, it holds the potential to make this highly effective adjuvant more widely accessible, ultimately lowering vaccine costs.
By introducing genes from the soap bark tree and other organisms into yeast, synthetic biologists have created a pathway to produce QS-21, an influential adjuvant in vaccines. The chemical makeup of QS-21 is shown in the background. Credit: Bianca Susara, Berkeley Lab
“During the pandemic, there was significant concern among public health officials about the availability of QS-21 adjuvant, as it relied on a single tree,” explained Jay Keasling, a professor at UC Berkeley and senior faculty scientist at Berkeley Lab. “From a global health perspective, there is a strong demand for an alternative source of this adjuvant.”
The production of QS-21 involved the insertion of 38 different genes from six organisms into yeast — marking one of the lengthiest biosynthetic pathways ever transplanted into an organism, according to Keasling.
“The production of the potent vaccine adjuvant QS-21 in yeast showcases the potential of synthetic biology to tackle major environmental and human health challenges,” said former UC Berkeley postdoctoral fellow Yuzhong Liu, the lead author of the study and currently an assistant professor at Scripps Research in La Jolla, California.
The findings will be featured in the May 8 edition of the journal Nature.
Expanding on Malaria Research
The use of adjuvants in vaccines was initially recognized in the 1920s, when alum, an aluminum salt, was found to enhance the effectiveness of a diphtheria vaccine. Alum has since been incorporated into numerous vaccines that utilize a component of a pathogen to stimulate immunity. By improving vaccine efficacy, adjuvants also enable physicians to administer smaller doses of the active ingredient, known as an antigen.
Not long after alum was found to increase the effectiveness of vaccines, a group of soap-like molecules was also discovered to have the same effect. In the 1960s, researchers focused on an extract of the Chilean soapbark tree (Quillaja saponaria) that strongly activates different parts of the immune system to enhance the impact of giving a vaccine antigen alone.Quillaja saponaria) has been one of the primary non-aluminum adjuvants in vaccines for the past 25 years and has been tested in over 120 clinical trials. It is present in the shingles vaccine (Shingrix) given to older adults, a malaria vaccine (Mosquirix) used in children to protect against the parasite Plasmodium falciparum, and the Novavax SARS-COVID-19 vaccine. Today, QS-21 is produced by extracting bark from the tree and chemically separating its various compounds, some of which are toxic. Although QS-21 is a complex molecule containing a terpene core and eight sugar molecules, it has been created in the lab. However, this synthesis involves 79 separate steps, starting from an intermediate chemical that also needs to be synthesized.Keasling, the CEO of the U.S. Department of Energy-funded Joint BioEnergy Institute (JBEI) in Emeryville, Calif., was asked to attempt to replicate the synthesis process in yeast. He has spent years adding genes to yeast to produce terpene compounds, including artemisinin, an antimalarial drug, as well as scents and flavorings. Terpene compounds, like those responsible for the scent of pine trees, are often fragrant.
He mentioned, “This work builds on our malaria work. We worked on the malaria therapy. Now, this could be an adjuvant for the malaria vaccines in the future.”
Adding the eight sugars was challenging, as was balancing unexpected interactions among enzymes in yeast. All of this had to be achieved without disrupting critical metabolic pathways necessary for yeast growth.
He expressed, “It has eight sugars and a terpenoid in the middle. I mean, it makes the artemisinin biosynthetic pathway look like nothing. I am pleased that synthetic biology has advanced to the point where we can now create a pathway to produce a molecule like QS-21. It demonstrates how much the field has progressed in the past two decades.”
He and his lab colleagues, led by postdoctoral fellow Liu, collaborated closely with plant researcher Anne Osbourn at the John Innes Center in the United Kingdom. Osbourn had previously identified the many enzymatic steps involved in the soapbark tree’s production of natural QS-21. Over the past five years, as Osbourn discovered new steps in the process and tested them in tobacco plants, Keasling’s lab gradually added these new genes to yeast to replicate the synthetic steps.
He mentioned, “It was a great collaboration, because as soon as she’d get a new gene in the pathway, they’d send it our way, and we’d put it into yeast.”
‘Everything From a Single Sugar’
Earlier this year, Osbourn and Keasling
published the full 20-step procedure
that the soapbark tree uses to make QS-21, recreated in tobacco. Unfortunately, tobacco is good for studying plant chemistry, but not for making a lot of a chemical compound. The new paper recreates that procedure in yeast, adding extra steps because yeast don't have some enzymes that plants do. Right now, a liter of the bioengineered yeast can make about 100 micrograms of QS-21 in three days, worth about $200. But using yeast to make it can be scaled up. “Even at the levels we’re making it, it’s cheaper than making it from the plant,” Keasling said.
The modified yeast only need sugar to survive, which is an added advantage, he said.
“My whole idea is, I want to make everything from a single sugar. I just want to feed yeast glucose, because eventually we want this process to be scaled. And if you feed them a bunch of fancy intermediates, then it’s going to result in a process that is not scalable,” Keasling said. “In the end, I’d like to start with glucose, so when the production is performed in large tanks, they’re able to produce QS-21 as easily and inexpensively as possible.”
While Keasling plans to leave optimization of the process for large-scale production to others, he does hope to adjust the enzymatic steps he has added to yeast to make versions of QS-21 that might be more effective than QS-21. And using yeast to make it lets him try cutting down the QS-21 molecule to see which parts can be removed without changing the molecule’s effectiveness.
Reference: “Complete biosynthesis of QS-21 in engineered yeast” 8 May 2024,
DOI: 10.1038/s41586-024-07345-9
The research was financially supported by an industrial grant. Nature.
A multistep biosynthetic process in yeast cheaply produces an expensive adjuvant now extracted from tree bark. Vaccines save lives, as proven during the recent pandemic,…
The research was funded by an industrial grant.
