Mating reaction inSaccharomyces cerevisiae

The effect of proteolytic enzymes on sexual agglutinability of haploid cells of the yeastSaccharomyces cerevisiae was examined. Sexual agglutinability of cells of botha and α types was lost on treatment with alkaline protease and two kinds of neutral proteases ofBacillus subtilis, pronase and α-chymotrypsin. Agglutinability of α type cells was lost after treatment with acid protease ofRhizopus chinensis and trypsin, but that ofa type cells was not. These results indicate that the sex-specific substance responsible for the sexual agglutination (agglutination factor) ina type cells differs from that in α type cells. Agglutination factors were solubilized from cell-wall fractions of both mating types by Glusulase treatment. These crude factors specifically inhibited the agglutinability of cells of the opposite mating type with little effect on the agglutinability of cells of the same mating type.     

Isolation and amino acid sequence of a mating pheromone produced by mating type α cells of Saccharomyces exiguus

A peptide, termed α^se pheromone, was isolated as a mating pheromone from culture filtrate of mating type a cells of Saccharomyces exiguus. The peptide showed both agglutinability-inducing activity to a cells of S. cerevisiae and shmoo-inducing action to a cells of S. cerevisiae, S. kluyveri and S. exiguus. The amino acid sequence of α^se pheromone was determined as H-Trp-His-Trp-Leu-Arg-Leu-Ser-Tyr-Gly-Gln-Pro-Ile-Tyr-OH by mass spectrometry, sequence analysis and enzymatic digestion.     

The mechanism of regulation of fibroblastic cell replication. III. A central role for nutrient limitation: a conditioning effect due to serum imprinting of the cell response

Haploid Saccharomyces cerevisiae cells of mating type a, but not α, produce and secrete a diffusible substance, designated a factor. The a factor transiently arrests cells of mating type α, but not a, at a very early stage of the cell cycle, prior to budding and to the initiation of DNA synthesis. While the cells are arrested at this stage, few, if any, of the functions required for the ensuing cell cycle are carried out. This stage of the cell cycle coincides with the stage at which α factor, produced by cells of mating type a, specifically arrests cells of mating type a [2]. It seems probable that the reciprocally acting a and α factors together provide the mechanism by which haploid cells are synchronized to the appropriate stage of the cell cycle as a prelude to conjugation.     

The Schizosaccharomyces pombe mam1 gene encodes an ABC transporter mediating secretion of M-factor

In the fission yeast Schizosaccharomyces pombe, cells of opposite mating type communicate via diffusible peptide pheromones prior to mating. We have cloned the S. pombe mam1 gene, which encodes a 1336-amino acid protein belonging to the ATP-binding cassette (ABC) superfamily. The mam1 gene is only expressed in M cells and the gene product is responsible for the secretion of the mating pheromone, M-factor, a nonapeptide that is S-farnesylated and carboxy-methylated on its C-terminal cysteine residue. The predicted Mam1 protein is highly homologous to mammalian multiple drug-resistance proteins and to the Saccharomyces cerevisiae STE6 gene product, which mediates export of a-factor mating pheromone. We show that STE6 can also mediate secretion of M-factor in S. pombe.     

White Cells Facilitate Opposite- and Same-Sex Mating of Opaque Cells in Candida albicans

Modes of sexual reproduction in eukaryotic organisms are extremely diverse. The human fungal pathogen Candida albicans undergoes a phenotypic switch from the white to the opaque phase in order to become mating-competent. In this study, we report that functionally- and morphologically-differentiated white and opaque cells show a coordinated behavior during mating. Although white cells are mating-incompetent, they can produce sexual pheromones when treated with pheromones of the opposite mating type or by physically interacting with opaque cells of the opposite mating type. In a co-culture system, pheromones released by white cells induce opaque cells to form mating projections, and facilitate both opposite- and same-sex mating of opaque cells. Deletion of genes encoding the pheromone precursor proteins and inactivation of the pheromone response signaling pathway (Ste2-MAPK-Cph1) impair the promoting role of white cells (MTL a) in the sexual mating of opaque cells. White and opaque cells communicate via a paracrine pheromone signaling system, creating an environment conducive to sexual mating. This coordination between the two different cell types may be a trade-off strategy between sexual and asexual lifestyles in C. albicans.     

Booklist no. 52 June 1984

When plus and minus mating type gametes of Chlamydomonas eugametos were mixed, a rapid transient increase in the amount of cAMP was observed with a maximum at 20 s after the start of the sexual agglutination reaction. The transient increase only occurred within the cells and was also exhibited when cell suspensions of single mating type were presented with isolated flagella of the other mating type. Cyclic AMP-dependent protein kinase and cyclic AMP-phosphodiesterase activities were found in cell homogenates. Since the rise in cAMP concentration preceded all known morphological and physiological changes in the cells that prepare them for fusion, it might be a primary response, induced by sexual agglutination.     

Interconversion of Yeast Mating Types III. Action of the Homothallism (HO) Gene in Cells Homozygous for the Mating Type Locus

Mating type interconversion in homothallic Saccharomyces cerevisiae has been studied in diploids homozygous for the mating type locus produced by sporulation of a/a/a/α and a/a/α/α tetraploid strains. Mating type switches have been analyzed by techniques including direct observation of cells for changes in α-factor sensitivity. Another method of following mating type switching exploits the observation that a/α cells exhibit polar budding and a/a and α/α cells exhibit medial budding.—These studies indicate the following: (1) The allele conferring the homothallic life cycle (HO) is dominant to the allele conferring the heterothallic life cycle (ho). (2) The action of the HO gene is controlled by the mating type locus—active in a/a and α/α cells but not in a/α cells. (3) The HO (or HO-controlled) gene product can act independently on two mating type alleles located on separate chromosomes in the same nucleus. (4) A switch in mating type is observed in pairs of cells, each of which has the same change.     

Interconversion of Yeast Mating Types II. Restoration of Mating Ability to Sterile Mutants in Homothallic and Heterothallic Strains

The two mating types of the yeast Saccharomyces cerevisiae can be interconverted in both homothallic and heterothallic strains. Previous work indicates that all yeast cells contain the information to be both a and α and that the HO gene (in homothallic strains) promotes a change in mating type by causing a change at the mating type locus itself. In both heterothallic and homothallic strains, a defective α mating type locus can be converted to a functional a locus and subsequently to a functional α locus. In contrast, action of the HO gene does not restore mating ability to a strain defective in another gene for mating which is not at the mating type locus. These observations indicate that a yeast cell contains an additional copy (or copies) of α information, and lead to the "cassette" model for mating type interconversion. In this model, HMa and hmα loci are blocs of unexpressed α regulatory information, and HMα and hma loci are blocs of unexpressed a regulatory information. These blocs are silent because they lack an essential site for expression, and become active upon insertion of this information (or a copy of the information) into the mating type locus by action of the HO gene.     

Interconversion of Yeast Mating Types I. Direct Observations of the Action of the Homothallism (HO) Gene

The HO gene promotes interconversion between a and α mating types. As a consequence, homothallic diploid cells are formed by mating between siblings descended from a single α HO or a HO spore. In order to determine the frequency and pattern of the mating-type switch, we have used a simple technique by which the mating phenotype can be assayed without losing the cell to the mating process itself. Specifically, we have performed pedigree analysis on descendants of single homothallic spores, testing these cells for sensitivity to α-factor.

The switch from α to a and vice versa is detectable after a minimum of two cell divisions. 50% of the clones tested showed switching by the four-cell stage. Of the four cells descended from a single cell, only the oldest cell and its immediate daughter are observed to change mating type. This pattern suggests that one event in the switching process has occurred in the first cell division cycle. Restriction of the switched mating-type to two particular cells may reflect the action of the homothallism system followed by nonrandom segregation of DNA strands in mitosis.

The mating behavior of cells which have sustained a change in mating type due to the HO gene is indistinguishable from that of heterothallic strains.

     

Pheromone communication in the fission yeast Schizosaccharomyces pombe

Conjugation between two haploid yeast cells is generally controlled by the reciprocal action of diffusible mating pheromones, cells of each mating type releasing pheromones that induce mating-specific changes in cells of the opposite type. Recent studies into pheromone signalling in the fission yeast Schizosaccharomyces pombe have revealed significant parallels with processes in higher eukaryotes and could provide the opportunity for investigating communication in an organism that is amenable to both biochemical and genetic manipulation.     

Mating Reaction in Saccharomyces cerevisiae. IV. Retardation of Deoxyribonucleic Acid Synthesis

When a and a type haploid cells of Saccharomyces cere-visiae were mixed and cultured, deoxyribonucleic acid synthesis was retarded but ribonucleic acid and protein syntheses were not. It was found that culture filtrate of a type cells inhibited deoxyribonucleic acid synthesis of a type cells and that of a type cells inhibited that of a type cells. Thus, sex-specific diffusible substances secreted by opposite mating type cells are thought, at least partly, to be responsible for the retardation of deoxyribonucleic acid synthesis.     

Morphological and Mutational Analysis of Mating in Ustilago hordei

Martinez-Espinoza, A. D., Gerhardt, S. A., and Sherwood, J. E. 1993. Morphological and mutational analysis of mating in Ustilago hordei. Experimental Mycology 17, 200-214. Ustilago hordei is a basidiomycete that causes covered smut on barley. Mating in U. hordei, which is controlled by a single locus with two alleles, results in the conversion of haploid, nonpathogenic yeast-like sporidia to dikaryotic, pathogenic mycelia. When sporidia of the opposite mating type were mixed and placed on water agar, both cell types produced conjugation tubes within 2 h at 21°C. Growth of conjugation tubes was directed toward the tip of tubes arising from cells of the opposite mating type. These tubes fused and the dikaryotic mycelium emerged from the conjugation bridge. Sporidia separated by a dialysis membrane were still capable of inducing conjugation tube formation by cells of the opposite mating type, indicating the involvement of diffusible small-molecular-weight mating factors (pheromones). Numerous nutritional and environmental variables were examined in order to optimize conjugation tube induction. Twenty-six mutants that fail to form dikaryotic mycelium have been isolated and characterized. These mutants were arranged into classes based on their ability to form conjugation tubes, the ability to induce conjugation tube formation by opposite mating-type cells, and cell morphology. These mutants provide an indication of the genetic complexity involved in this critical phase of the U. hordei life cycle.     

Mechanisms that ensure monogamous mating in Saccharomyces cerevisiae

Haploid cells of the budding yeast Saccharomyces cerevisiae communicate using secreted pheromones and mate to form diploid zygotes. Mating is monogamous, resulting in the fusion of precisely one cell of each mating type. Monogamous mating in crowded conditions, where cells have access to more than one potential partner, raises the question of how multiple-mating outcomes are prevented. Here we identify mutants capable of mating with multiple partners, revealing the mechanisms that ensure monogamous mating. Before fusion, cells develop polarity foci oriented toward potential partners. Competition between these polarity foci within each cell leads to disassembly of all but one focus, thus favoring a single fusion event. Fusion promotes the formation of heterodimeric complexes between subunits that are uniquely expressed in each mating type. One complex shuts off haploid-specific gene expression, and the other shuts off the ability to respond to pheromone. Zygotes able to form either complex remain monogamous, but zygotes lacking both can re-mate.     

Reversible arrest of haploid yeast cells in the initiation of DNA synthesis by a diffusible sex factor

A diffusible substance, α factor, is produced constitutively by haploid yeast cells of α mating type and this factor specifically inhibits the division of a mating type cells. Experiments are presented which demonstrate that α factor arrests a cells as unbudded, mononucleate cells prior to the initiation of DNA synthesis in the cell cycle. Studies with temperature-sensitive mutants defective in one of thirteen different cell cycle functions suggest that although arrested a cells continue to enlarge they do not perform functions required for the next cell cycle. The arrest is reversible and a partially synchronized round of DNA replication is observed upon removal of α factor from arrested cells. We propose that this factor is one element of a regulatory system that functions to assure the synchronization of a and α haploid cell cycles prior to conjugation.     

Serologic grouping and sexual compatibility of airbornecryptococcus neoformans

Through the use of Anderson air samplers, 214 isolates ofCryptococcus neoformans were cultured from the air in a vacant tower in a large complex of buildings in Oklahoma City. The tower contained hundreds of pigeons, a massive amount of droppings, nests with eggs and young, dying and dead pigeons. All isolates were serotype A-D and self-sterile for the production of basidiospores. Among these, 193 were of the ‘alpha’ mating type, producing basidiospores when paired with ‘a’ mating type. No isolates of ‘α’ mating type were found. The remaining 21 isolates were untypable for their mating type. These findings imply that the infectious particles ofC. neoformans in nature are relatively small, nonencapsulated yeast cells andnot basidiospores.     

Nonsex Genes in the Mating Type Locus of Candida albicans Play Roles in a/α Biofilm Formation,Including Impermeability and Fluconazole Resistance

The mating type locus (MTL) of Candida albicans contains the mating type genes and has, therefore, been assumed to play an exclusive role in the mating process. In mating-incompetent a/α cells, two of the mating type genes, MTL a1 and MTLα2, encode components of the a1-α2 corepressor that suppresses mating and switching. But the MTL locus of C. albicans also contains three apparently unrelated “nonsex” genes (NSGs), PIK, PAP and OBP, the first two essential for growth. Since it had been previously demonstrated that deleting either the a/α copy of the entire MTL locus, or either MTLa1 or MTLα2, affected virulence, we hypothesized that the NSGs in the MTL locus may also play a role in pathogenesis. Here by mutational analysis, it is demonstrated that both the mating type and nonsex genes in the MTL locus play roles in a/α biofilm formation, and that OBP is essential for impermeability and fluconazole resistance.     

Sex specificity of hormone synthesis in Mucor mucedo

Sex specificity is observed in the mating types of the fungus Mucor mucedo with respect to the production of 4-hydroxy methyltrisporates (plus mating type) and trisporins (minus mating type), and in the conversion of these metabolites to trisporic acids by the mating partner. These compounds induce zygophores on the opposite mating type only.