While most organisms are routinely exposed to sunlight, and sunlight is necessary to sustain much life, the ultraviolet radiation it emits also harms living cells, causing damage to membranes, DNA and other cellular components. Ultraviolet (UV) radiation damages a cell’s DNA by causing a change in a nucleotide sequence, also known as a mutation. Cells are able to repair some of this damage on their own. However, if the damage is not repaired before the cell divides, the mutation will be passed on to the new cells. Studies show that longer exposure to UV radiation results in higher levels of mutation and cell death; these effects are more severe the longer a cell is exposed.
Why Do We Care About Yeast?
Yeast are single-celled micro-organisms, but the genes responsible for DNA repair are very similar to those of a human. In fact, they share a common ancestor around a billion years ago and have 23 percent of their genes in common. Like human cells, yeast are eukaryotic organisms; they have a nucleus that contains DNA. Yeast is also easy to work with and inexpensive, making it an ideal specimen to determine the effects of radiation on cells.
Humans and yeast also have a symbiotic relationship. Our intestinal tracts are home to more than 20 species of yeast-like fungi. Candida albicans, the most common, has been a frequent subject of study. While usually harmless, an overgrowth of this yeast can trigger infections in certain body parts, most commonly the mouth or throat (known as thrush) and the vagina (also referred to as a yeast infection). In rare cases, it may enter the bloodstream, where it can spread through the body and cause dangerous infections. It can also spread to other patients; for this reason it is considered a global health threat. Researchers are looking to regulate the growth of this yeast using a light-sensitive switch to prevent resulting fungal infections.
The ABCs of Ultraviolet Radiation
While the most common source of ultraviolet radiation is sunlight, some artificial lights also emit ultraviolet radiation. Under normal conditions, incandescent lights (ordinary light bulbs) emit only a small amount of ultraviolet light, although more is emitted at higher intensities. While quartz-halogen lamps (commonly used for automotive headlights, overhead projectors and outdoor lighting) emit a greater amount of damaging ultraviolet light, these bulbs are usually enclosed in glass, which absorbs some of the dangerous rays.
Fluorescent lights emit photon energy, or UV-C waves. These lights are enclosed in tubes that allow very little of the UV waves to escape. Different coating materials can change the range of photon energy emitted (e.g., black lights emit UV-A waves). A germicidal lamp is a specialized device that produces UV-C rays and is the only common UV source capable of disrupting the normal yeast repair systems. While UV-C rays have been investigated as a potential treatment for infections caused by Candida, they are limited in use since they also damage surrounding host cells.
Exposure to UV-A radiation provides humans with necessary vitamin D, but these rays can penetrate deep into skin layers and cause sunburn, premature aging of the skin, cancer or even suppression of the body’s immune system. Damage to the eye is also possible, which can lead to cataracts. UV-B radiation mostly affects the skin’s surface. It is absorbed by DNA and the ozone layer and causes the skin to increase production of the pigment melanin, which darkens the skin. It is the primary cause of sunburn and skin cancer. UV-C is the most damaging type of radiation, but since it is completely filtered by the atmosphere, it is rarely a concern for humans.
Cellular Changes in DNA
Unlike ionizing radiation (the type seen in X-rays and when exposed to radioactive materials), ultraviolet radiation does not break covalent bonds, but it does make limited chemical changes to DNA. There are two copies of each kind of DNA per cell; in many cases, both copies must be damaged in order to kill the cell. Ultraviolet radiation often only damages one.
Ironically, light can be used to help repair damage to cells. When UV-damaged cells are exposed to filtered sunlight, enzymes in the cell use the energy from this light to reverse the reaction. If these lesions are repaired before the DNA tries to replicate, the cell remains unchanged. However, if the damage is not repaired before the DNA replicates, the cell may suffer “reproductive death.” In other words, it may still be able to grow and metabolize, but will be unable to divide. At exposure to higher levels of radiation, the cell may suffer metabolic death, or die completely.
Effects of Ultraviolet Rays on Yeast Colony Growth
Yeast are not solitary organisms. Though they are single-celled, they exist in a multicellular community of interacting individuals. Ultraviolet radiation, in particular UV-A rays, negatively impacts colony growth, and this damage increases with prolonged exposure. While ultraviolet radiation has been proven to cause damage, scientists have also found ways to manipulate light waves to improve the efficiency of UV-sensitive yeast. They have found that light causes more damage to yeast cells when they are actively respiring and less damage when they are fermenting. This discovery has led to new ways of manipulating the genetic code and maximizing the use of light to influence cellular processes.
Optogenetics and Cellular Metabolism
Through a research field called optogenetics, scientists use light-sensitive proteins to regulate a variety of cellular processes. By manipulating cells' exposure to light, researchers have discovered that different colors of light can be used to activate different proteins, cutting down the time necessary for some chemical productions. Light has benefits over chemical or pure genetic engineering. It is inexpensive and works faster, and the function of the cells is easy to turn on and off as the light is manipulated. Unlike chemical adjustments, light can be applied to only specific genes rather than affecting the entire cell.
After adding light-sensitive genes to yeast, researchers trigger or suppress the activity of genes by manipulating the light available to the genetically modified yeast. This results in an increase in the output of certain chemicals and broadens the scope of what can be produced through yeast fermentation. In its natural state, yeast fermentation produces high volumes of ethanol and carbon dioxide, and trace amounts of isobutanol, an alcohol used in plastics and lubricants, and as an advanced biofuel. In the natural fermentation process, isobutanol at high concentrations kills off entire yeast colonies. However, using the light-sensitive, genetically modified strain, researchers prompted the yeast to produce amounts of isobutanol up to five times higher than previously reported levels.
The chemical process that allows for yeast growth and replication only happens when the yeast is exposed to light. Since the enzymes that produce isobutanol are inactive during the fermentation process, the desired alcohol product is only produced in the dark, so the light must be shut off for them to do their job. By using intermittent bursts of blue light every few hours (just enough to keep them from dying), the yeast produces higher amounts of isobutanol.
Similarly, Saccharomyces cerevisiae naturally produces shikimic acid, which is used in several medications and chemicals. While ultraviolet radiation often damages yeast cells, scientists added a modular semiconductor to the metabolic machinery of yeast to provide biochemical energy. This changed the yeast’s central metabolism, allowing cells to increase production of shikimic acid.
- PNAS: Visible Light Alters Yeast Metabolic Rhythms by Inhibiting Respiration
- University of Wisconsin-Madison: Building a Biological Control Switch With Light, Genetics, and Engineering Ingenuity
- Princeton University: Researchers Use Light to Turn Yeast Into Biochemical Factories
- The Harvard Gazette: Yeasts Get a Boost From Solar Power
- NASA: Radiation Damage in Living Organisms
- Science: Light-Driven Fine Chemical Production in Yeast Biohybrids
- Found of Biology and Scientific Inquiry Lab: The Effect of UV Light Exposure on Saccharomyces Cerevisiae (Baker's Yeast) and the Efficacy of Different Color Plastic Wrap in Protecting Against UV Radiation
- Cells: The Yeast Saccharomyces Cerevisiae as a Model for Understanding RAS Proteins and Their Role in Human Tumorigenesis
- Virulence: Antimicrobial Blue Light Inactivation of Candida Albicans: In Vitro and in Vivo Studies