How do yeast control the expression of their genes? And how is this relevant to beer and wine fermentation? On this page we’ll dive into the details of gene expression regulation, and learn how specific DNA sequences, called promoters, can impact the flavor of fermented beverages.
What is a promoter?
The DNA that makes up every gene can be divided into different functional segments (Figure 1). One segment, called the “coding sequence”, contains the information necessary for producing a protein. A second segment, called the promoter, is located next to the coding sequence and determines precisely when the gene will be expressed, and how many proteins will be produced from it. Essentially, a promoter is responsible for sensing the physiological state of the cell (e.g., nutrient levels, temperature, and pH), and using this information to determine what the appropriate gene expression level should be. Every one of yeast’s approximately 7,000 genes has its own unique promoter, which means that every yeast gene has its own unique level of expression that is specific to that gene’s function.
Figure 1. The ATF1 promoter and coding sequence.
As discussed in our lesson on gene expression regulation, the expression level of a gene will change in response to its environment. These expression changes are controlled by the gene’s promoter. For simplicity, scientists often characterize promoters on a sliding scale from strong to weak. Strong promoters are those that, in most environments, drive high levels of gene expression, resulting in many proteins being produced. Conversely, weak promoters are those that generally drive low levels of expression, resulting in few proteins being produced.
Why are promoters important for beer and winemaking?
Many of the differences between various brewing and winemaking strains result from differences in the regulation of gene expression. For example, brewers have long known that Hefeweizen strains produce more isoamyl acetate (banana-like flavor) than other ale strains. Scientists recently discovered that this is because Hefeweizen strains strongly express the ATF1 gene, which encodes an isoamyl acetate biosynthesizing enzyme¹. The reason that Hefeweizen strains highly express ATF1 is likely that these strains have mutations in their ATF1 promoter that increases the level of ATF1 expression (Figure 2).
Aside from the production of isoamyl acetate, many other traits that differ among yeast strains result from differences in the regulation of gene expression. Traits like fermentation speed, flocculation, and volatile thiol biotransformation are all affected by mutations in promoter sequences that change the strength and timing of gene expression. These and other discoveries have made it clear that changes in gene expression can have a huge impact on yeast fermentation performance and the flavor of fermented beverages.
Figure 2. A strong ATF1 promoter results in beer with banana-like flavors.
Researchers at Berkeley Yeast have found that modifying gene expression levels can also be a key strategy in creating yeast strains with enhanced fermentation traits. By making targeted changes to the DNA sequence of a gene’s promoter, we can precisely control the timing and strength of that gene’s expression. This fine degree of genetic control can be incredibly useful in creating strains that produce specific concentrations of flavor molecules. You can learn more about this in our discussion of how we bioengineer strains.
1. Schneiderbanger, H., Koob, J., Poltinger, S., Jacob, F. & Hutzler, M. Gene expression in wheat beer yeast strains and the synthesis of acetate esters. J. Inst. Brew. 122, 403–411 (2016).