The term “gene expression”, refers to the process by which the information in genes is used to create proteins. When scientists say that a gene is being expressed, what they mean is that the information encoded in that gene is being used as a template for producing many identical proteins.
Yeast cells tightly control which genes are being expressed at any given time. This makes sense, as cells do not want to waste energy expressing genes that are not useful in their current environment. To give an example, consider a yeast cell that was just pitched into a beer fermentation and is now surrounded by the wort sugars glucose, maltose, and maltotriose. This cell will quickly activate the expression of hundreds of genes that enable fermentation of these sugars. This set of genes will encode a variety of different proteins, ranging from transporters that bring sugar molecules into the cell, to enzymes like kinases and dehydrogenases that convert the sugars into energy and ethanol. After several days of fermentation however, the yeast will have consumed all of the wort sugars. Other nutrients, like amino acids and essential minerals will also have become scarce. In response to this new environment, the yeast will alter the expression levels of key genes: Expression of sugar transporters, kinases, and dehydrogenases will be reduced, while the expression of starvation response genes —encoding amino acid biosynthesis and nutrient scavenging enzymes— will be increased (Figure 1).
Figure 1. Gene expression in different environments.
The process of dynamically regulating which genes are being expressed is critical in enabling yeast to survive in different environments. How are yeast able to achieve this precise level of regulation over gene expression? And how is this relevant to creating better brewing and winemaking strains? To answer these questions, continue on to the next lesson and dive a bit deeper into the biology of gene expression regulation.