The determination of mating types in yeast is a genetic mechanism that determines the manifestation of a particular type of mating in a particular yeast cell [1] .
This mechanism has been studied most thoroughly in Baker's yeast - Saccharomyces cerevisiae . In S. cerevisiae , both diploid and haploid ( ascospore ) stages are present in the life cycle. The determination of mating types occurs only in haploid forms. In total there are two types of mating: a and α. The type of mating is regulated by a single locus - MAT , responsible for the characteristics characteristic of each sex type. During each cell cycle, a haploid cell can change sex by a special form of genetic recombination — see below ] [2] [3] . The switching mechanism of mating types in S. cerevisiae was proposed in the late 1970s. XX century. A. Herskowitz, J. Hicks and J. Strasner [1] .
Another type of yeast is Schiz. pombe [1] - a sex switching mechanism was also found, similar to that in S. cerevisiae . Some basidiomycetes have similar sex determination mechanisms [4] , but they have many alleles responsible for the sexual type, not just two, and the number of mating types goes up to several thousand [5] .
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Mating types and the life cycle of S. cerevisiae
As stated above, S. cerevisiae can exist in the haploid and diploid stages. Both forms are able to multiply by budding , which is preceded by mitosis , and the daughter cell buds from the mother. Haploid cells, in addition, are able to fuse with other haploid cells of the opposite type of mating ( a -cages with α-cells, and vice versa), forming a stable diploid cell. Diploid cells, as a rule, under stressful conditions (for example, depletion of nutrient resources) can undergo meiosis, giving rise to four haploid ascospores: two a- spores and two α-spores [3] .
Differences between a - and α-cells
a -cells secrete a -factor, pheromone , signaling the presence of a -cells to neighboring α-cells. A- cells, in turn, respond to the α-factor, α-cell pheromone, expanding towards the source of the α-factor. Α-cells behave in a similar way. This ensures the mating of only haploid cells of different types of mating and makes it impossible to mate cells of the same sex type [6] [2] .
Phenotypic differences between a - and α-cells are due to active transcription and suppression of different sets of genes . Thus, in α- cells, the genes responsible for the synthesis of α-factor are active, as well as genes that activate the formation of surface cell receptors (STE2), which bind to the α-factor and signal the presence of α-cells to the whole cell. A cells also suppress the genes responsible for the phenotypic manifestations of the a-type. Α-cells act in a similar way (their surface cell receptors are designated as STE3) [2] .
Suppression of expression of certain sets of genes at the transcription stage and activation of others, characteristic of a - and α-cells, are due to the presence of one of the two alleles of the MAT: MAT a and MATα locus. The MAT a allele contains the a1 gene, which in a haploid cell determines the launch of an a -specific transcriptional program (for example, STE2 expression and suppression of STE3 expression), which determines the phenotype of the a -cell. The MATα allele encompasses the α1 and α2 genes, which in a haploid cell determines the launch of an α-specific transcriptional program (for example, STE3 expression and suppression of STE2 expression), which determines the α-cell phenotype.
Differences between haploid and diploid cells
Haploid cells have one of two sex types ( a or α), respond to pheromones of cells of the opposite sex type and can mate with them. Haploid cells cannot undergo meiosis . Diploid cells do not form pheromones and do not react to them; they do not mate, but can divide by meiosis to form four haploid cells .
Like the differences between haploid a - and α-cells, the different activation and repression of different sets of genes are also responsible for the phenotypic differences between haploid and diploid cells. In addition to both a- and α-specific transcriptional programs, both types of haploid cells have a specific transcriptional program that activates the expression of haploid-specific genes (for example, HO) and suppresses the expression of diploid cell-specific genes (for example, IME1). Similarly, diploid cells activate diploid-specific genes and suppress haploid-specific ones.
The expression of different genes in haploids and diploids is again due to the MAT locus. Haploid cells contain only one set of 16 chromosomes and therefore they can have only one allele of the MAT locus (or MAT a , or MATα), which determines their sexual type. Diploid cell is formed as a result of mating of a - and α-cells, and therefore contains 32 chromosomes in 16 pairs, including one chromosome with a MAT a a- allele and one with a MATα-allele. The combination of information encoded by the MAT a allele ( a1 gene) and the MATα allele (α1 and α2 genes) starts a transcriptional program specific for diploid cells. Similarly, the presence of only one allele MAT-MAT a or MAT- α starts a haploid-specific transcriptional program.
The MAT locus allele contains enough information for the sexual behavior of cells. For example, with the help of genetic manipulations, the MAT a allele can be added to the haploid α-cell with the MATα allele. Despite the fact that the cell is haploid in the remaining loci, it now contains both MATα and MAT a and diploid at MAT, and therefore will behave like a diploid cell: it will not produce pheromones and react to them, and when fasting it will try to share meiosis, which will lead to a fatal result due to haploidy on the remaining genes. Similarly, if one MAT allele is removed from a diploid cell, leaving either the MATα allele, or MAT a , then the diploid cell for other genes will behave like a haploid cell.
Switching mating type
Mated wild-type yeast capable of switching mating type between a and α. Therefore, even if there is only one haploid cell of a certain type of mating in the colony of yeast, switching the sex will result in the presence of both a - and α-type cells in the population. Due to the persistent stimulus of haploid cells to mate with haploid cells of the opposite sex type, sex switching and subsequent mating will lead to the fact that most of the cells in the colony are diploid, regardless of whether they are diploid or haploid, the cells were first in it. The overwhelming majority of yeast strains studied in laboratories were altered so much that they lost the ability to switch sex (due to the deletion of the HO gene; see below). This ensures stable reproduction of haploid cells, since haploid cells of a- type will remain a- cells (and cells of an α-type - α-cells) and will not give diploids among themselves.
HMR and HML: silent cassettes
Haploid yeasts switch the field, replacing the information contained in the MAT locus. For example, a cells can turn into α cells, replacing the MAT a allele with MAT α . Such replacement of one MAT allele by another is possible due to the fact that yeast cells contain an additional silent copy of both alleles: the HML locus (from the English Hidden MAT Left ) usually carries a silent copy of the MATα allele and is located to the left of the MAT locus, and the HMR usually (from the English . Hidden MAT Right ) contains a silent copy of MAT a and is located to the right of MAT. Silent loci HMR and HML are often called silent cassettes, since their information is transmitted to the active locus MAT (therefore, the yeast sex switching mechanism itself is called cassette [1] ).
Additional copies of the alleles responsible for mating types do not interfere with the functioning of the MAT locus, no matter what allele it contains, because they are not expressed, and the haploid cell with the MAT a allele at the MAT locus remains a -cell, despite the presence of the silent MATα allele in HML locus. Only the allele enclosed in the MAT locus is transcribed, and only it affects the behavior of the cell.
Mating type switching mechanism
The mating type switching process is essentially gene conversion initiated by the BUT gene. The BUT gene is strictly regulated by a haploid-specific gene that is activated only in haploid cells in the G 1 phase of the cell cycle. The BUT gene encodes a DNA enzyme , the endonuclease , which cuts DNA exclusively in the region of the MAT locus (since BUT-endonuclease is specific for the DNA sequence of this region).
As soon as the HO-endonuclease cuts DNA at the MAT locus, exonucleases are attracted to the ends of the DNA being cut and begin to destroy the DNA on both sides of the cutting site. This degradation of DNA by exonucleases removes the DNA contained in the MAT allele. However, this gap is repaired using DNA copied from HML or HMR, and as a result, the MAT a or MATα allele is inserted. Thus, the silent alleles of MAT a and MATα, which are present in the HML and HMR loci, serve as a source of genetic information for the repair of BUT-induced DNA damage in the MAT active locus. The cells prefer to change the type of mating, i.e. the a -cell is more likely to insert the MATα allele into the rupture and become an α-cell, and vice versa. The mechanism for this specificity is unknown.
Directivity of mating type switching
Reparation of the MAT-locus after cutting with HO-endonuclease almost always leads to a change in the type of mating. When the a cell cuts the MAT a allele at the MAT locus, the MAT is almost always repaired with information from the HML. Thus, the MAT will be repaired with the MATα allele, and the mating type will change from a to α. Similarly, in the α-cell, repair is almost always carried out from the HMR locus, because of which the MAT in a MAT appears and the mating type switches from α to a .
The reason for this error lies in the recombination enhancer ( born recombination enhancer (RE) ) [7] , located on the left shoulder of chromosome III. The deletion of this region causes incorrect reparation in the a- cells, in which a copy of the HMR is inserted at the site of rupture. Normally, in a- cells, the Mcm1 transcription factor is associated with RE, thereby triggering a conversion with copying and pasting a copy of HML. In α-cells, the α2 factor binds to the RE and overlaps it with its one domain , repressing it and thereby suppressing the conversion. In fact, it is shown that the most problematic is reparation with HMR. The specific mechanisms of the described processes are still being established.
Regulation
The information contained in the cassettes (that is, the HMRa and HMLα genes) is not expressed in haploid cells due to the presence of special sequences — silencers ( silencers ) that affect the nature of chromatin compaction in cassettes. Silencers are controlled by special SIR genes located on other chromosomes. There are no silencers near the MAT locus [1] .
Notes
- ↑ 1 2 3 4 5 Inge-Vechtomov, 2010 , p. 518-520.
- ↑ 1 2 3 Beth A Montelone. Yeast Mating Type // ENCYCLOPEDIA OF LIFE SCIENCES. - 2002. - DOI : 10.1038 / npg.els.0000598 . (inaccessible link)
- ↑ 1 2 Harvey Lodish, Arnold Berk, S Lawrence Zipursky, Paul Matsudaira, David Baltimore, James Darnell. Molecular Cell Biology. - 4th edition. - New York: WH Freeman, 2000. - ISBN 978-0-7167-3136-3 .
- ↑ Jacques Labarère, Thierry Noel. Mating type switching in teterapolar basidiomycete Agrocybe aegerita // Genetics society of America. - 1992.
- ↑ Lorna A. Casselton, Natalie S. Olesnicky. Molecular Genetics of Mating Recognition in Basidiomycete Fungi // Microbiology and molecular biology reviews. - 1998. - № 62 .
- ↑ Jane B. Reece, Lisa A. Urry, Michael L. Cain et. al. Campbell: Biology. - 9th edition .. - Benjamin Cummings. - P. 206-207. - 1263 p. - ISBN 978-0-321-55823-7 .
- ↑ The Saccharomyces cerevisiae recombination enhancer biases recombination during interchromosomal mating-type switching homologous recombination (English) // Genetics: journal. - 2004. - March ( vol. 166 , no. 3 ). - P. 1187-1197 . - PMID 15082540 .
Literature
- Inge-Vechtomov S.G. Genetics with the basics of selection. - SPb. : Publisher NL, 2010. - 718 p. - ISBN 987-5-94869-105-3.