General Introduction
Most of us know yeast is a very helpful organism, especially with respect to baking, wine making,
and brewing. However, what are yeast and why are they the focus of so much research?
Introduction to FungiCredit: Kandis Elliot, Mo Fayyaz, UW-Madison
Yeast are Fungi
Yeast are single-celled microorganisms that are classified, along with molds and mushrooms, as
members of the Kingdom Fungi. Yeasts are evolutionally diverse and are therefore classified into
two separate phyla, Ascomycota or sac fungi and Basidiomycota or higher fungi, that together form
the subkingdom Dikarya. Budding yeast, also referred to as “true yeasts”, are members of the
phylum Ascomycota and the order Saccharomycetales. Such classifications are based on
characteristics of the cell, ascospore, and colony, as well as cellular physiology.
Yeast are Single-celled, but with Cellular Organization Similar to Higher Organisms
Yeast are single-celled organisms classified as eukaryotes due the presence of a nucleus that harbors their
genetic information. Credit: Wikicommons
Although yeast are single-celled organisms, they possess a cellular organization similar to that of
higher organisms, including humans. Specifically, their genetic content is contained within a nucleus.
This classifies them as eukaryotic organisms, unlike their single-celled counterparts, bacteria, which
do not have a nucleus and are considered prokaryotes.
Natural Habitats
Yeast are widely dispersed in nature with a wide variety of habitats. They are commonly found on
plant leaves, flowers, and fruits, as well as in soil. Yeast are also found on the surface of the skin and
in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites.
The common "yeast infection" is typically caused by Candida albicans. In addition to being the
causative agent in vaginal yeast infections, Candida is also the cause of diaper rash and thrush of the
mouth and throat
Yeast Life and Cell Cycles
Budding yeast life cycle. Credit: Wikicommons
Yeast typically grow asexually by budding. A small bud which will become the daughter cell is
formed on the parent (mother) cell, and enlarges with continued grow. As the daughter cell grows,
the mother cell duplicates and then segregates its DNA. The nucleus divides and migrates into the
daughter cell. Once the bud contains a nucleus and reaches a certain size it separates from the mother
cell. The series of events that occur in a cell and lead to duplication and division are referred to as the
cell cycle. The cell cycle consists of four distinct phases (G1, S, G2 and M) and is regulated similar
to that of the cell cycle in larger eukaryotes. As long as adequate nutrients such as sugar, nitrogen and
phosphate are present yeast cells will continue to divide asexually.
Shmooing yeast cell.Credit: Wikicommons
Yeast cells can also reproduce sexually. Yeast cells exist as one of two different mating types, a cells
and alpha cells. When cells of opposite mating types are mixed together in the lab or randomly come
into contact in nature they can mate (conjugate). Before joining the cells change shape in a process
called shmooing. The term 'shmoo' was coined based on its similarity in shape to that of a fictional
cartoon character of the same name created back in the late 40's by Al Capp, appearing first in his
comic strip L'il Abner. During conjugation the shmooing haploid cells first fuse and then their nuclei
fuse, resulting in the formation of a diploid cell with two copies of each chromosome. Once formed,
diploid cells can reproduce asexually by budding, similar to haploids. However, when diploid cells
are starved of nutrients they undergo sporulation. During sporulation diploid cells undergo meiosis, a
special form of cell division that reduces the number of chromosomes from two copies back to one
copy. After meiosis the haploid nuclei produced in meiosis are packaged into four spores that contain
modified cell walls, resulting in structures that are very resistant to environmental stress. These
spores can survive long periods of time until conditions become more favorable, such as in the
presence of improved nutrients, whereupon they are able to germinate and reproduce asexually.
These different states, budding, conjugation and sporulation together make up the yeast life cycle.
CO2 bubbles produced during fermentation. Credit: Wikicommons
Yeast growth and metabolism
When yeast cells are grown in rich carbon sources such as glucose they prefer to grow by
fermentation. During fermentation glucose is converted into carbon dioxide and ethanol. Generally,
fermentation occurs in the absence of oxygen, and is therefore anaerobic by nature. Even in the
presence of oxygen yeast cells prefer to grow fermentatively and this is referred to as the Crabtree
Effect after the biologist who discovered this preference. This form of growth is exploited in the
making of bread, beer, wine and other alcoholic beverages. Although budding yeast cells prefer to
grow by fermentation, when nutrients are limiting they are also able to grow by cellular respiration.
During respiration cells convert glucose into carbon dioxide and water, consuming oxygen in the
process, and resulting in the production of much larger amounts of energy in the form of ATP.
Historical Discoveries
Egyptian wooden model of beer making in ancient Egypt. Credit: Wikicommons
Yeast has been used as an industrial microorganism for 1000’s of years. The ancient Egyptians used
yeast fermentation to leaven bread. There is evidence of grinding stones, baking chambers and
drawings of 4000-year-old bakeries. Archaeological digs have uncovered evidence in the form of jars
containing the remains of wine that is 7,000 years old.
Yeast were first visualized in 1680 by Antoni van Leeuwenhoek using high quality lenses. However,
he thought that these globules were starchy particles of the grain used to make wort, the liquid extract
used in brewing, rather than fermenting yeast cells. In 1789, Antoine Lavoisier, a French chemist,
contributed to our understanding of the basic chemical reactions needed to produce alcohol from
sugarcane. By estimating the proportion of starting materials and products (ethanol and carbon
dioxide) after adding yeast paste he concluded that two chemical pathways were used with two thirds
of the sugar reduced to alcohol and one third to form carbon dioxide. However, at the time it was
thought that yeast were merely there to initiate the reaction rather than being required throughout the
process.
Ascus of S. cerevisiae containing a tetrad of four spores. Credit: Wikicommons
In 1815, Joseph-Louis Gay-Lussac, a French chemist, developed methods to maintain grape juice in
an unfermented state and discovered that the introduction of ‘ferment’ (which contains yeast) was
required to convert unfermented wort, demonstrating the importance of yeast for alcoholic
fermentation. In 1835, Charles Cagniard de la Tour used a more powerful microscope to show that
yeast were single celled and multiplied by budding. In the 1850s Louis Pasteur discovered that
fermented beverages resulted from the conversion of glucose to ethanol by yeast and defined
fermentation as "respiration without air". Near the end of the 1800s Eduard Buchner used cell-free
extracts obtained by grinding yeast cells to detect zymase, the collection of enzymes that promote or
catalyze fermentation and for this he was awarded the Nobel Prize in 1907.
Much of the pioneering work on yeast genetics was carried out by Øjvind Winge. He discovered that
yeast alternate between haploid and diploid states and that yeast are heterothallic, as two strains are
required to convert haploids to diploids (conjugation). He and his colleague Otto Laustsen devised
techniques to micromanipulate yeast so they could be investigated genetically. With this technique,
known as "tetrad analysis", a fine needle and a microscope are used to isolate a structure known as an
ascus, which contains the four spore products or tetrad resulting from sporulation of a diploid. Once
the ascus is isolated, the spores in the tetrad are teased apart and allowed to grow into colonies for
genetic analysis. This pioneering work earned him the title ‘The Father of Yeast Genetics’. Some of
this work was further clarified by Carl Lindegren, who elucidated the mating-type system in budding
yeast, demonstrating the existence of Mat a and Mat alpha cells, devised methods to carry our mass
matings between cells of these mating types and used this knowledge to study the genetics of sugar
utilization.
Since that time many other researchers have carried out groundbreaking research using budding
yeast. Some of these researchers have been awarded the Nobel Prize for significant discoveries made
during these studies, including: Dr. Leland Hartwell (2001) for the discovery of genes that regulate
the cell cycle (co-winner with Paul Nurse and Tim Hunt); Roger Kornberg (2006) for his studies on
the first step of gene expression, the means by which a genes DNA sequence is copied into
messenger RNA (mRNA); Drs. Elizabeth Blackburn, Carol Greider and Jack Szostak (2009) for
discovering and elucidating the genes and means by which cells protect chromosome ends or
telomeres from being degraded; and to Drs. Randy Schekman, James Rothman and Thomas Südhof
(2013) for research on the machinery that regulates vesicular traffic. Most recently, Dr. Yoshinori
Ohsumi was awarded the prize for his work on autophagy, which began with studies in yeast.
Commercial Applications
Yeast is used to make beer and bread. Credit: Wikicommons
Yeast has long been considered to be the organism of choice for the production of alcoholic
beverages, bread, and a large variety of industrial products. This is based on the ease with which the
metabolism of yeast can be manipulated using genetic techniques, the speed with which it can be
grown to high cell yields (biomass), the ease with which this biomass can be separated from products
and the knowledge that it is generally recognized as safe (GRAS).
The budding yeast S. cerevisiae and other yeast species have long been used to ferment the sugars of
rice, wheat, barley, and corn to produce alcoholic beverages such as beer and wine. There are two
major types of brewing yeast, top-fermenting ale yeast and bottom-fermenting lager yeast. Topfermenting yeast such as S. cerevisiae rise to the surface during fermentation and are used to brew
ales, porters, stouts and wheat beers. In contrast, S. pastorianus, (formerly known as S.
carlsbergensis) is a bottom-fermenting yeast used to make lager beer. Lager yeasts grow best at
lower temperatures. As a result they grow more slowly, produce less surface foam, and therefore
typically settle to the bottom of the fermenter. Pilsners, Märzen, Bocks, and American malt liquors
are all styles of lager beer. In modern brewing many of the original top fermenting strains have been
modified to become bottom fermenters.
Yeast produce wine by fermenting sugars from grape juice (must) into ethanol. Although wine
fermentation can be initiated by naturally occurring yeast present in the vineyards, many wineries
choose to add a pure yeast culture to dominate and control the fermentation. The bubbles in
champagne and sparkling wines are produced by a secondary fermentation, typically in the bottle,
which traps the carbon dioxide. Carbon dioxide produced in wine production is released as a byproduct. One yeast cell can ferment approximately its own weight in glucose per hour. Under optimal
conditions S. cerevisiae can produce up to 18 percent, by volume of ethanol with 15 - 16% being the
norm. The sulfur dioxide present in commercially produced wine is added just after the grapes are
crushed to kill the naturally present bacteria, mold, and yeast.
Yeast has a nutty, cheesy flavor making it an ideal cheese substitute. Credit: Wikicommons
Saccharomyces cerevisiae or baker’s yeast has long been used as a leavening agent in baking.
Baker’s yeast ferment sugars present in dough, producing carbon dioxide and ethanol. The carbon
dioxide becomes trapped in small bubbles in the dough, which causes the dough to rise. Sourdough
bread is an exception, as it is not produced using baker's yeast, but is instead made with a
combination of wild yeast and bacteria. The yeast Candida milleri is used to strengthen the gluten,
and an acid-generating bacteria “Lactobacillus sanfranciscensis”, is used to ferment the maltose.
In addition to these traditional uses yeast has also been used for many other commercial applications.
Vegans often use yeast as a cheese substitute and it is often used as a topping for products such as
popcorn. It is being used in the petrochemical industry where it has been engineered to produce
biofuels such as ethanol, and farnesene, a diesel and jet fuel precursor. It is also used in the
production of lubricants and detergents. Yeast is used in the food industry for the production of food
additives including colorants, antioxidants, and flavor enhancers. It is the often used in the
production of pharmaceuticals including antiparasitics, anticancer compounds, biopharmaceuticals
such as insulin, vaccines, and nutraceuticals. Yeast is commonly used in the production of industrial
enzymes and chemicals. In the field of environmental bioremediation strains have even been
exploited for the removal of metal from mining waste.
Application to Human Disease and Research
By virtue of the high degree of similarity between yeast genes and their human counterparts, and
conserved fundamental cellular biology, yeast has become a popular model system for the study of
human disease genes. Several approaches have been used to learn more about human genes once a
connection between a human and yeast gene is made. In one approach, after a human diseaseassociated gene is discovered the sequence is compared to the sequences of all genes in the yeast
genome to identify the most similar yeast gene(s). To study whether the genes are functionally
related, the human gene is then expressed in a yeast stain where the yeast gene has first been
inactivated by mutation. This allows researchers to determine whether or not the human gene is able
to rescue viability, growth, or more specific defects associated with loss of the yeast gene, a method
referred to as functional complementation. If the pathways and/or processes that a yeast gene is
involved in are conserved, much can be learned about the function of the human gene based on what
is already known about the related yeast gene. Once functional complementation has been
established, researchers can use this system to further characterize the function of the related human
gene product. Less directed approaches that often utilize high-throughput (HTP) techniques to
randomly screen thousands of human genes at one time to identify gene or genes with
complementing activity. Such approaches have successfully been used to identify conserved cell
cycle regulators (CDC2), genes involved in cancer, and genes involved in neurodegenerative
diseases.
There are many scenarios where studies can provide valuable information to researchers about the
cellular pathways and/or processes a human gene is involved in when a related yeast gene is not
present. For example, some neurodegenerative diseases like Alzheimer’s and Parkinson’s occur as
protein aggregates called amyloid accumulate due to protein misfolding and this is toxic to neurons.
Studying misfolded yeast proteins with similar amyloid forming potential, called prions, has provided
researchers with insight into these neurodegenerative diseases. Alternatively, elevated expression of a
disease-associated gene in yeast may result in a phenotype. For example, when expressed at high
enough levels, alpha-synuclein, a gene associated with Parkinson’s disease, is toxic. Such a strain can
then be used to screen for yeast genes or small molecules that suppress or enhance synuclein-induced
toxicity, often providing clues about the relevant cellular pathways. Patients with Amyotrophic
Lateral Sclerosis (ALS) or Lou Gehrig’s Disease, often have mutations in a couple of RNA binding
proteins which makes them prone to form aggregates that interfere with RNA metabolism. A yeast
screen has been used successfully to identify a number of yeast genes with similar properties (form
toxic aggregates) providing researchers with new candidate genes to study. Conversely, when
expressed in yeast the human RNA binding proteins form toxic aggregates and this strain was used to
identify a yeast gene which when mutated blocks the production of these aggregates.
Yeast is becoming the organism of choice in studies aimed at the identification of drug targets and
the mode of action of various drugs. Chemogenomics or chemical-genomics refers to the screens that
use a combination of chemicals and genomics to probe drug targets and potentially identify novel
drugs. Two main approaches have been used in these chemical-genomic studies. In the first, a
genome-wide collection of diploid strains is constructed where one of the two identical copies of a
gene is deleted, thereby lowering the levels of a particular gene product. Target genes and genes
involved in the target pathway become more sensitive to the compound and are preferentially
identified in this kind of screen. In a second approach, nonessential genes are systematically deleted
and the collection screened with a drug to look for genes which buffer the drug target pathway. This
approach is expected to identify genes required for growth in the presence of the compound.
Additional approaches using overexpression screens have been used to identify genes involved in
drug resistance including the potential drug target. Comparing the expression profile of yeast cells
deleted for a gene to those of wild type yeast cells treated with a particular drug can also be an
effective way to identify genes which may tell the researchers something about how the drug works
in cells.
These are just a few examples of how yeast can be used both aid the study of human disease. Studies
in yeast can help researchers learn more about the underlying biology using this model system, or to
help them identify drug targets or the drugs mode of action.