Mating of yeast
Encyclopedia
The yeast Saccharomyces cerevisiae
is a simple single celled
eukaryote
with both a diploid and haploid mode of existence. The mating of yeast only occurs between haploids, which can be either the a or α (alpha) mating type and thus display simple sexual differentiation
. Mating type is determined by a single locus
, MAT, which in turn governs the sexual behaviour of both haploid and diploid cells. Through a form of genetic recombination
, haploid yeast can switch mating type as often as every cell cycle
.
, with daughter cells budding off of mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type (an a cell can only mate with an α cell, and vice versa) to produce a stable diploid cell. Diploid cells, usually upon facing stressful conditions such as nutrient depletion, can undergo meiosis
to produce four haploid spore
s: two a spores and two α spores.
a cells produce ‘a-factor’, a mating pheromone
which signals the presence of an a cell to neighbouring α cells. a cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo
, due to its distinctive shape) towards the source of α-factor. Similarly, α cells produce α-factor, and respond to a-factor by growing a projection towards the source of the pheromone. The response of haploid cells only to the mating pheromones of the opposite mating type allows mating between a and α cells, but not between cells of the same mating type.
These phenotypic
differences between a and α cells are due to a different set of gene
s being actively transcribed
and repressed in cells of the two mating types. a cells activate genes which produce a-factor and produce a cell surface receptor
(Ste2) which binds to α-factor and triggers signaling
within the cell. a cells also repress the genes associated with being an α cell. Similarly, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to a-factor, and α cells repress the genes associated with being an a cell.
The different sets of transcriptional repression and activation which characterize a and α cells are caused by the presence of one of two allele
s of a locus
called MAT: MATa or MATα. The MATa allele of MAT encodes a gene called a1, which in haploids direct the transcription of the a-specific transcriptional program (such as expressing STE2 and repressing STE3) which defines an a cell. The MATα allele of MAT encodes the α1 and α2 genes, which in haploids direct the transcription of the α-specific transcriptional program (such as expressing STE3, repressing STE2) which causes the cell to be an α cell.
. Diploid cells do not produce or respond to either mating pheromone and do not mate, but can undergo meiosis
to produce four haploid cells.
Like the differences between haploid a and α cells, different patterns of gene repression and activation are responsible for the phenotypic
differences between haploid and diploid cells. In addition to the specific a and α transcriptional patterns, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid-specific genes (such as HO) and represses diploid-specific genes (such as IME1). Similarly, diploid cells activate diploid-specific genes and repress haploid-specific genes.
The different gene expression patterns of haploids and diploids are again due to the MAT locus. Haploid cells only contain one copy of each of the 16 chromosome
s and thus can only possess one allele of MAT (either MATa or MATα), which determines their mating type. Diploid cells result from the mating of an a cell and an α cell, and thus possess 32 chromosomes (in 16 pairs), including one chromosome bearing the MATa allele and another chromosome bearing the MATα allele. The combination of the information encoded by the MATa allele (the a1 gene) and the MATα allele (the α1 and α2 genes) triggers the diploid transcriptional program. Similarly, the presence of only a single allele of MAT, whether it is MATa or MATα, triggers the haploid transcriptional program.
The alleles present at the MAT locus are sufficient
to program the mating behaviour of the cell. For example, using genetic manipulations
, a MATa allele can be added to a MATα haploid cell. Despite having a haploid complement of chromosomes, the cell now has both the MATa and MATα alleles, and will behave like a diploid cell: it will not produce or respond to mating pheromones, and when starved will attempt to undergo meiosis, with fatal results. Similarly, deletion of one copy of the MAT locus in a diploid cell, leaving only a single MATa or MATα allele, will cause a cell with a diploid complement of chromosomes to behave like a haploid cell.
Wild type haploid yeast are capable of switching mating type between a and α. Consequently, even if a single haploid cell of a given mating type founds a colony
of yeast, mating type switching will cause cells of both a and α mating types to be present in the population. Combined with the strong drive for haploid cells to mate with cells of the opposite mating type and form diploids, mating type switching and consequent mating will cause the majority of cells in a colony to be diploid, regardless of whether a haploid or diploid cell founded the colony. The vast majority of yeast strains
studied in laboratories
have been altered such that they cannot perform mating type switching (by deletion of the HO gene; see below); this allows the stable propagation of haploid yeast, as haploid cells of the a mating type will remain a cells (and α cells will remain α cells), and will not form diploids.
copy of both the MATa and MATα alleles: the HML (Hidden MAT Left) locus typically carries a silenced copy of the MATα allele, and the HMR (Hidden MAT Right) locus typically carries a silenced copy of the MATa allele. The silent HML and HMR loci are often referred to as the silent mating cassettes, as the information present there is 'read into' the active MAT locus.
These additional copies of the mating type information do not interfere with the function of whatever allele is present at the MAT locus because they are not expressed, so a haploid cell with the MATa allele present at the active MAT locus is still an a cell, despite also having a (silenced) copy of the MATα allele present at HML. Only the allele present at the active MAT locus is transcribed, and thus only the allele present at MAT will influence cell behaviour.
The process of mating type switching is a gene conversion
event initiated by the HO gene. The HO gene is a tightly regulated haploid-specific gene that is only activated in haploid cells during the G1 phase
of the cell cycle
. The protein
encoded by the HO gene is a DNA endonuclease
, which physically cleaves DNA, but only at the MAT locus (due to the DNA sequence specificity of the HO endonuclease).
Once HO cuts the DNA at MAT, exonuclease
s are attracted to the cut DNA ends and begin to degrade the DNA on both sides of the cut site. This DNA degradation by exonucleases eliminates the DNA which encoded the MAT allele; however, the resulting gap in the DNA is repaired
by copying in the genetic information present at either HML or HMR, filling in a new allele of either the MATa or MATα gene. Thus, the silenced alleles of MATa and MATα present at HML and HMR serve as a source of genetic information to repair the HO-induced DNA damage at the active MAT locus. The cells prefere to change the mating type, i.e. a MATa cell will rather use HMLα to fill the gap thus become MATα and vice versa. The mechanism for this specificity is unknown.
This is the result of the action of a recombination enhancer (RE) located on the left arm of chromosome III. Deletion of this region causes a cells to incorrectly repair using HMR. In a cells, Mcm1 binds to the RE and promotes recombination of the HML region. In α cells, the α2 factor binds at the RE and establishes a repressive domain over RE such that recombination is unlikely to occur. An innate bias means that the default behaviour is repair from HMR. The exact mechanisms of these interactions are still under investigation.
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...
is a simple single celled
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
with both a diploid and haploid mode of existence. The mating of yeast only occurs between haploids, which can be either the a or α (alpha) mating type and thus display simple sexual differentiation
Sexual differentiation
Sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote...
. Mating type is determined by a single locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
, MAT, which in turn governs the sexual behaviour of both haploid and diploid cells. Through a form of genetic recombination
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
, haploid yeast can switch mating type as often as every cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
.
Mating type and the life cycle of Saccharomyces cerevisiae
S. cerevisiae (yeast) can stably exist as either a diploid or a haploid. Both haploid and diploid yeast cells reproduce by mitosisMitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, with daughter cells budding off of mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type (an a cell can only mate with an α cell, and vice versa) to produce a stable diploid cell. Diploid cells, usually upon facing stressful conditions such as nutrient depletion, can undergo meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
to produce four haploid spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s: two a spores and two α spores.
Differences between a and α cells
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
which signals the presence of an a cell to neighbouring α cells. a cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo
Shmoo
A shmoo is a fictional cartoon creature. Created by Al Capp , it first appeared in his classic comic strip Li'l Abner on August 31, 1948, and quickly became a postwar national craze in the USA....
, due to its distinctive shape) towards the source of α-factor. Similarly, α cells produce α-factor, and respond to a-factor by growing a projection towards the source of the pheromone. The response of haploid cells only to the mating pheromones of the opposite mating type allows mating between a and α cells, but not between cells of the same mating type.
These phenotypic
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
differences between a and α cells are due to a different set of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s being actively transcribed
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
and repressed in cells of the two mating types. a cells activate genes which produce a-factor and produce a cell surface receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
(Ste2) which binds to α-factor and triggers signaling
Cell signaling
Cell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue...
within the cell. a cells also repress the genes associated with being an α cell. Similarly, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to a-factor, and α cells repress the genes associated with being an a cell.
The different sets of transcriptional repression and activation which characterize a and α cells are caused by the presence of one of two allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s of a locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
called MAT: MATa or MATα. The MATa allele of MAT encodes a gene called a1, which in haploids direct the transcription of the a-specific transcriptional program (such as expressing STE2 and repressing STE3) which defines an a cell. The MATα allele of MAT encodes the α1 and α2 genes, which in haploids direct the transcription of the α-specific transcriptional program (such as expressing STE3, repressing STE2) which causes the cell to be an α cell.
Differences between haploid and diploid cells
Haploid cells are one of two mating types (a or α), and respond to the mating pheromone produced by haploid cells of the opposite mating type, and can mate with cells of the opposite mating type. Haploid cells cannot undergo meiosisMeiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. Diploid cells do not produce or respond to either mating pheromone and do not mate, but can undergo meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
to produce four haploid cells.
Like the differences between haploid a and α cells, different patterns of gene repression and activation are responsible for the phenotypic
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
differences between haploid and diploid cells. In addition to the specific a and α transcriptional patterns, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid-specific genes (such as HO) and represses diploid-specific genes (such as IME1). Similarly, diploid cells activate diploid-specific genes and repress haploid-specific genes.
The different gene expression patterns of haploids and diploids are again due to the MAT locus. Haploid cells only contain one copy of each of the 16 chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s and thus can only possess one allele of MAT (either MATa or MATα), which determines their mating type. Diploid cells result from the mating of an a cell and an α cell, and thus possess 32 chromosomes (in 16 pairs), including one chromosome bearing the MATa allele and another chromosome bearing the MATα allele. The combination of the information encoded by the MATa allele (the a1 gene) and the MATα allele (the α1 and α2 genes) triggers the diploid transcriptional program. Similarly, the presence of only a single allele of MAT, whether it is MATa or MATα, triggers the haploid transcriptional program.
The alleles present at the MAT locus are sufficient
Necessary and sufficient conditions
In logic, the words necessity and sufficiency refer to the implicational relationships between statements. The assertion that one statement is a necessary and sufficient condition of another means that the former statement is true if and only if the latter is true.-Definitions:A necessary condition...
to program the mating behaviour of the cell. For example, using genetic manipulations
Genetic engineering
Genetic engineering, also called genetic modification, is the direct human manipulation of an organism's genome using modern DNA technology. It involves the introduction of foreign DNA or synthetic genes into the organism of interest...
, a MATa allele can be added to a MATα haploid cell. Despite having a haploid complement of chromosomes, the cell now has both the MATa and MATα alleles, and will behave like a diploid cell: it will not produce or respond to mating pheromones, and when starved will attempt to undergo meiosis, with fatal results. Similarly, deletion of one copy of the MAT locus in a diploid cell, leaving only a single MATa or MATα allele, will cause a cell with a diploid complement of chromosomes to behave like a haploid cell.
Mating type switching
Clonal colony
A clonal colony or genet is a group of genetically identical individuals that have grown in a given location, all originating vegetatively from a single ancestor. In plants, an individual in such a population is referred to as a ramet...
of yeast, mating type switching will cause cells of both a and α mating types to be present in the population. Combined with the strong drive for haploid cells to mate with cells of the opposite mating type and form diploids, mating type switching and consequent mating will cause the majority of cells in a colony to be diploid, regardless of whether a haploid or diploid cell founded the colony. The vast majority of yeast strains
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...
studied in laboratories
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...
have been altered such that they cannot perform mating type switching (by deletion of the HO gene; see below); this allows the stable propagation of haploid yeast, as haploid cells of the a mating type will remain a cells (and α cells will remain α cells), and will not form diploids.
HML and HMR: the silent mating cassettes
Haploid yeast switch mating type by replacing the information present at the MAT locus. For example, an a cell will switch to an α cell by replacing the MATa allele with the MATα allele. This replacement of one allele of MAT for the other is possible because yeast cells carry an additional silencedGene silencing
Gene silencing is a general term describing epigenetic processes of gene regulation. The term gene silencing is generally used to describe the "switching off" of a gene by a mechanism other than genetic modification...
copy of both the MATa and MATα alleles: the HML (Hidden MAT Left) locus typically carries a silenced copy of the MATα allele, and the HMR (Hidden MAT Right) locus typically carries a silenced copy of the MATa allele. The silent HML and HMR loci are often referred to as the silent mating cassettes, as the information present there is 'read into' the active MAT locus.
These additional copies of the mating type information do not interfere with the function of whatever allele is present at the MAT locus because they are not expressed, so a haploid cell with the MATa allele present at the active MAT locus is still an a cell, despite also having a (silenced) copy of the MATα allele present at HML. Only the allele present at the active MAT locus is transcribed, and thus only the allele present at MAT will influence cell behaviour.
Mechanics of the mating type switch
Gene conversion
Gene conversion is an event in DNA genetic recombination, which occurs at high frequencies during meiotic division but which also occurs in somatic cells. It is a process by which DNA sequence information is transferred from one DNA helix to another DNA helix, whose sequence is altered.It is one...
event initiated by the HO gene. The HO gene is a tightly regulated haploid-specific gene that is only activated in haploid cells during the G1 phase
G1 phase
The G1 phase is a period in the cell cycle during interphase, before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great...
of the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
. The protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
encoded by the HO gene is a DNA endonuclease
Endonuclease
Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain, in contrast to exonucleases, which cleave phosphodiester bonds at the end of a polynucleotide chain. Typically, a restriction site will be a palindromic sequence four to six nucleotides long. Most...
, which physically cleaves DNA, but only at the MAT locus (due to the DNA sequence specificity of the HO endonuclease).
Once HO cuts the DNA at MAT, exonuclease
Exonuclease
Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end of a polynucleotide chain. A hydrolyzing reaction that breaks phosphodiester bonds at either the 3’ or the 5’ end occurs. Its close relative is the endonuclease, which cleaves phosphodiester bonds in the middle ...
s are attracted to the cut DNA ends and begin to degrade the DNA on both sides of the cut site. This DNA degradation by exonucleases eliminates the DNA which encoded the MAT allele; however, the resulting gap in the DNA is repaired
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
by copying in the genetic information present at either HML or HMR, filling in a new allele of either the MATa or MATα gene. Thus, the silenced alleles of MATa and MATα present at HML and HMR serve as a source of genetic information to repair the HO-induced DNA damage at the active MAT locus. The cells prefere to change the mating type, i.e. a MATa cell will rather use HMLα to fill the gap thus become MATα and vice versa. The mechanism for this specificity is unknown.
Directionality of the mating type switch
The repair of the MAT locus after cutting by the HO endonuclease almost always results in a mating type switch. When an a cell cuts the MATa allele present at the MAT locus, the cut at MAT will almost always be repaired by copying the information present at HML. This results in MAT being repaired to the MATα allele, switching the mating type of the cell from a to α. Similarly, an α cell which has its MATα allele cut by the HO endonuclease will almost always repair the damage using the information present at HMR, copying the MATa gene to the MAT locus and switching the mating type to a.This is the result of the action of a recombination enhancer (RE) located on the left arm of chromosome III. Deletion of this region causes a cells to incorrectly repair using HMR. In a cells, Mcm1 binds to the RE and promotes recombination of the HML region. In α cells, the α2 factor binds at the RE and establishes a repressive domain over RE such that recombination is unlikely to occur. An innate bias means that the default behaviour is repair from HMR. The exact mechanisms of these interactions are still under investigation.
External links
- Fungi Can Tell Us About The Origin Of Sex Chromosomes: study shows that there are great similarities between the parts of DNA that determine the sex of plants and animals and the parts of DNA that determine mating types in certain fungi. Accessed 5 April 2008.