For thousands of years, humanity has harnessed the unique abilities of yeast to brew alcohol and bake bread. These microorganisms act as nature's minuscule factories, transforming sugars found in fruits, grains, and other sources into alcohol for beverages and releasing carbon dioxide to make bread rise. Today, researchers at the School of Engineering at Tufts University are giving yeast a new job description. Findings from this new research were published recently in Metabolic Engineering.

In a step forward for the promising field of synthetic biology, these scientists have engineered a type of yeast capable of feeding on a broader range of substances, including underutilized agricultural by-products such as leaves, husks, stems, and wood chips. These materials, often classified as "waste biomass," present an untapped resource in a world increasingly concerned with sustainable solutions.

The researchers' pioneering work aims to answer a critical question: why is it essential to produce yeast that can subsist on these agricultural leftovers? The answer lies in the multi-faceted potential of genetically modified yeast. Scientists have already harnessed this organism to manufacture useful products such as biofuels and pharmaceuticals, using a technique that does not require harmful chemicals.

Tufts' groundbreaking research has led to a yeast strain capable of consuming non-traditional sugars such as xylose, arabinose, and cellobiose. These sugars can be extracted from the inedible woody parts of crops that are typically discarded post-harvest, like corn stalks, husks, leaves, and wheat stems.

“By getting yeast to feed on waste biomass, we can potentially create a biosynthetic industry with a significantly reduced carbon footprint," said Associate Professor Nikhil Nair, part of the research team at Tufts. "When we burn biofuels made by yeast, we produce a lot of carbon dioxide. But that carbon dioxide is reabsorbed into crops the following year, which the yeast feeds on to make more biofuel, creating a sustainable cycle."

Sean Sullivan, a graduate student in the Nair lab who co-led the recent study, emphasized the importance of designing a single yeast organism capable of digesting a complete menu of biomass sugars. The team accomplished this by making minimal alterations to the existing genetic 'dashboard' or 'regulon' that yeast uses to regulate sugar consumption.

Another co-leader of the study, post-doctoral researcher Vikas Trivedi, highlighted the benefits of their 'minimal engineering approach,' noting, "It turns out that yeast has the machinery to grow on non-native sugars, as long as we adapt sensors and regulons to recognize those sugars."

Modifying yeast to feed on waste biomass opens the door to improved production of biosynthesized products, including insulin, human growth hormone, and antibodies. Additionally, yeast can produce vaccines and natural compounds used to make drugs, usually sourced from rare plants.

Furthermore, while yeast's role in producing ethanol is well-known, researchers have also enabled it to produce other fuels like isobutanol and isopentanol, which provide more energy per liter than ethanol. The engineered yeast can also manufacture the building blocks of bioplastics, reducing dependence on petroleum sources.

Nair concluded, "As the research community continues to innovate yeast to make new products, we are preparing the organism to grow efficiently on agricultural waste biomass, closing a carbon cycle that has so far eluded the manufacturing of fuels, pharmaceuticals, and plastics."