A mutation that permits the expression of normally silent copies of mating-type information in Saccharomyces cerevisiae

Studies of heterothallic and homothallic strains of Saccharomyces cerevisiae have led to the suggestion that mating-type information is located at three distinct sites on chromosome 3, although only information at the mating-type (MAT) locus is expressed (Hicks, Strathern and Herskowitz, 1977). We have found that the recessive mutation cmt permits expression of the normally silent copies of mating-type information at the HMa and HM alpha loci. In haploid strains carrying HMa and HM alpha, the cmt mutation allows the simultaneous expression of both a and alpha information, leading to a nonmating ("MATa/MAT alpha") phenotype. The effects of cmt can be masked by changing the mating-type information at HMa or HM alpha. For example, a cell of genotype MATa hma HM alpha cmt has an a mating type, while a MAT alpha hma HM alpha cmt strain is nonmating. Expression of mating-type information at the HM loci can correct the mating and sporulation defects of the mata* and mat alpha 10 alleles. Meiotic segregants recovered from cmt/cmt diploids carrying the mat mutations demonstrate that these mutants are not "healed" to normal MAT alleles, as is the case in parallel studies using the homothallism gene HO.--All of the results are consistent with the notion that the HMa and hm alpha alleles both code for alpha information, while HM alpha and hma both code for a information. The cmt mutation demonstrates that these normally silent copies of mating-type and sporulation information can be expressed and that the information at these loci is functionally equivalent to that found at MAT. The cmt mutation does not cause interconversions of mating-type alleles at MAT, and it is not genetically linked to MAT, HMa, HM alpha or HO. In cmt heterozygotes, cmt becomes homozygous at a frequency greater than 1% when the genotype at the MAT locus is mata*/MAT alpha or mat alpha 10/MATa.     

Mating-Type Differentiation by Transposition of Controlling Elements in SACCHAROMYCES CEREVISIAE

The nonfunctional mutation of the homothallic gene HML alpha, designated hml alpha, produced two mutant alleles, hml alpha-1 and hml alpha-2. Both mutant clones were mixed cultures consisting of a mating-type cells and nonmating haploid cells. The frequencies of the two cell types were different, and a few diploid cells able to sporulate were found in the hml alpha-2 mutant. Conversions of an a mating-type cell to nonmater, and vice versa, were observed in both mutants. The conversion of an a mating phenotype to nonmating is postulated to occur by alteration of the a mating type to the sterile mating-type allele in the hml alpha-1 mutant. In tetrad dissection of prototrophic diploids that were obtained by rare mating of hml alpha-1 mutants with a heterothallic strain having the MATa ho HMRa HMLa genotype, many mating-deficient haploid segregants were found, while alpha mating-type segregants were observed in a similar diploid using an hml alpha-2 mutant. The mating-type-deficient haploid segregants were supposed to have the sterile alpha mating-type allele because the nonmating genetic trait always segregated with the mating-type locus. Sporogenous diploid cells obtained in the hml alpha-2 mutant clone had the MATa/MAT alpha HO/HO HMRa/HMRa hml alpha-2/hml alpha-2 genotype. These observations suggested that the hml alpha-1 allele produces a transposable element that gives rise to the sterile alpha mating type by transposition into the mating-type locus, and that the hml alpha-2 allele produces an element that provides alpha mating-type information, but is defective in the structure for transposition.     

A Suppressor of Mating-Type Locus Mutations in SACCHAROMYCES CEREVISIAE: Evidence for and Identification of Cryptic Mating-Type Loci

A mutation has been identified that suppresses the mating and sporulation defects of all mutations in the mating-type loci of S. cerevisiae. This suppressor, sir1-1, restores mating ability to mat alpha 1 and mat alpha 2 mutants and restores sporulation ability to mat alpha 2 and mata1 mutants. MATa sir1-1 strains exhibit a polar budding pattern and have reduced sensitivity to alpha-factor, both properties of a/alpha diploids. Furthermore, sir1-1 allows MATa/MATa, mat alpha 1/mat alpha/, and MAT alpha/MAT alpha strains to sporulate efficiently. All actions of sir1-1 are recessive to SIR1. The ability of sir1-1 to supply all functions necessary for mating and sporulation and its effects in a cells are explained by proposing that sir1-1 allows expression of mating type loci which are ordinarily not expressed. The ability of sir1-1 to suppress the mat alpha 1-5 mutation is dependent on the HMa gene, previously identified as required for switching of mating types from a to alpha. Thus, as predicted by the cassette model, HMa is functionally equivalent to MAT alpha since it supplies functions of MAT alpha. We propose that sir1-1 is defective in a function. Sir ("Silent-information regulator"), whose role may be to regulate expression of HMa and HM alpha.     

Saccharomyces cerevisiae nuclear fusion requires prior activation by alpha factor.

We have developed a protocol for efficient fusion of spheroplasts of the same mating type. Nuclear fusion in this whole-cell system is also efficient and closely parallels nuclear fusion in heterosexual mating of intact cells. In the spheroplast fusion system, nuclear fusion is dependent on both the KAR1 gene and prior exposure to alpha factor. The major products of nuclear fusion in the spheroplast fusion assay were true diploids that were homozygous at the mating-type locus. An additional 10% of the products were cells of ploidy greater than diploid. The dependence of nuclear fusion on alpha factor treatment could not be replaced by synchronization in G1 by mutations in CDC28 and CDC35 or by prior arrest in stationary phase. These data suggest that nuclear fusion is not a constitutive function of the nucleus, but rather is specifically induced by mating hormone.     

Analysis of Y-Linked Mutations to Male Sterility in DROSOPHILA MELANOGASTER

Mating type in haploid cells of the yeast Saccharomyces cerevisiae is determined by a pair of alleles MATa and MAT alpha. Under various conditions haploid mating types can be interconverted. It has been proposed that transpositions of silent cassettes of mating-type information from HML OR HMR to MAT are the source of mating type conversions. A mutation described in this work, designated AON1, has the following properties. (1) MAT alpha cells carring AON1 are defective in mating. (2) AON1 allows MAT alpha/MAT alpha but not MATa/MATa diploids to sporulate; thus, AON1 mimics the MATa requirement for sporulation. (3) mata-1 cells that carry AON1 are MATa phenocopies, i.e., MAT alpha/mata-1 AON1 diploids behave as standard MAT alpha/MATa cells; therefore, AON1 suppresses the defect of mata-1. (4) AON1 maps at or near HMRa. (5) Same-site revertants from AON1 lose the ability to convert mating type to MATa, indicating that reversion is associated with the loss of a functional HMRa locus. In addition, AON1 is a dominant mutation. We conclude that AON1 is a regulatory mutation, probably cis-acting, that leads to the constitutive expression of silent a mating-type information located at HMRa.     

Isolation and genetic analysis of Saccharomyces cerevisiae mutants supersensitive to G1 arrest by a factor and alpha factor pheromones.

Eight independently isolated mutants which are supersensitive (Sst-) to the G1 arrest induced by the tridecapeptide pheromone alpha factor were identified by screening mutagenized Saccharomyces cerevisiae MATa cells on solid medium for increased growth inhibition by alpha factor. These mutants carried lesions in two complementation groups, sst1 and sst2. Mutations at the sst1 locus were mating type specific: MATa sst1 cells were supersensitive to alpha factor, but MAT alpha sst1 cells were not supersensitive to a factor. In contrast, mutations at the sst2 locus conferred supersensitivity to the pheromones of the opposite mating type on both MATa and MAT alpha cells. Even in the absence of added alpha pheromone, about 10% of the cells in exponentially growing cultures of MATa strains carrying any of three different alleles of sst2 (including the ochre mutation sst2-4) had the aberrant morphology ("shmoo" shape) that normally develops only after MATa cells are exposed to alpha factor. This "self-shmooing" phenotype was genetically linked to the sst2 mutations, although the leakiest allele isolated (sst2-3) did not display this characteristic. Normal MATa/MAT alpha diploids do not respond to pheromones; diploids homozygous for an sst2 mutation (MATa/MAT alpha sst2-1/sst2-1) were still insensitive to alpha factor. The sst1 gene was mapped to within 6.9 centimorgans of his6 on chromosome IX. The sst2 gene was unlinked to sst1, was not centromere linked, and was shown to be neither linked to nor centromere distal to MAT on the right arm of chromosome III.     

Mating Type and Sporulation in Yeast I. Mutations Which Alter Mating-Type Control over Sporulation

In Saccharomyces cerevisiae, meiosis and spore formation as well as mating are controlled by mating-type genes. Diploids heterozygous for mating type (aα) can sporulate but cannot mate; homozygous aa and αα diploids can mate, but cannot sporulate. From an αα diploid parental strain, we have isolated mutants which have gained the ability to sporulate. Those mutants which continue to mate as αα cells have been designated CSP (control of sporulation). Upon sporulation, CSP mutants yield asci containing 4α spores. The mutant gene which allows αα cells to sporulate is unlinked to the mating-type locus and also acts to permit sporulation in aa diploid cells. Segregation data from crosses between mutant αα and wild-type aa diploids and vice versa indicate (for all but one mutant) that the mutation which allows constitutive sporulation (CSP) is dominant over the wild-type allele. Some of the CSP mutants are temperature-sensitive, sporulating at 32°, but not at 23°. In addition to CSP mutants, our mutagenesis and screening procedure led to the isolation of mutants which sporulate by virtue of a change in the mating-type locus itself, resulting in loss of ability to mate.     

Alpha mating type-specific expression of mutations leading to constitutive agglutinability in Saccharomyces cerevisiae.

Two mutants of Saccharomyces cerevisiae have been isolated and characterized. The mutants were constitutively agglutinable at 36 degrees C, the temperature at which wild-type cells agglutinate only after induction by mating pheromone. The mutant cells had other properties specific for the normal alpha cell type, i.e., conjugation with a cells, response to a mating pheromone, and production of alpha mating pheromone. The two mutations, cag1 and cag2, were recessive and expressed only in alpha cells. cag1 is linked very closely to the MAT locus, but cag2 is unlinked to the MAT locus. These cag mutations complemented ste3-1. These results indicate that CAG genes are novel alpha-specific genes involved in the regulation of sex agglutinin synthesis.     

rme1 Mutation of Saccharomyces cerevisiae: map position and bypass of mating type locus control of sporulation.

Sporulation in Saccharomyces cerevisiae normally occurs only in MATa/MAT alpha diploids. We show that mutations in RME1 bypassed the requirements for both a and alpha mating type information in sporulation and therefore allowed MATa/MATa and MAT alpha/MAT alpha diploids to sporulate. RME1 was located on chromosome VII, between LEU1 and ADE6.     

Modification changes of the genetic material in Saccharomyces yeasts

The problem of mating-type switches in heterothallic yeast cells was investigated. In selective system for cytoduction in alpha x alpha crosses alpha-cytoductants were predominantly obtained. Thus matings in alpha x alpha crosses can proceed through non-heritable changes (modifications) of the mating type alpha----a. The frequency of alpha-cytoductants after UV-irradiation of the recipient cells exceeded the control value 50-90 times. The extra copy of MAT alpha dramatically decreased the frequency of cytoductants in alpha x alpha crosses, either spontaneously or after UV-irradiation. The rad18 recipient defective in postreplication repair had 70-times increased level of mating-type modifications, as compared with isogenic Rad+ strain. An explanation consistent with these data is that mating-type modifications are due to phenotypic expression of primary lesions of MAT alpha locus. Such lesions might be expressed as transient a-mating type. After the mating event, these lesions can be repaired or turned to true mutations within the MAT locus. In fact, approximately half of non-mating cytoductants from alpha x alpha crosses had the phenotype of mat alpha 2 mutants.     

Mating-Type Mutations in SCHIZOSACCHAROMYCES POMBE: Isolation of Mutants and Analysis of Strains with an h or h Phenotype

Mutants defective in various steps of the sexual cycle have been isolated from homothallic strains of Schizosaccharomyces pombe by Bresch, Müller and Egel (1968). These mutants include heterothallic h(+) and h(-) strains. We have isolated additional h(+) and h(- ) mutants from homothallic strains. Those mutants which are due to mutations in the mating-type region were analyzed in detail. Our results show that the mating-type gene mat2 not only has a function in copulation and meiosis, but that it also regulates the formation of the map1 gene product (map1 is a mating-type auxiliary gene). Some of the h( -) mutants have lost only one of the three functions while others are defective in at least two, and perhaps all three, functions. Further, we show that the mat1(-) allele of h(90) strains can mutate to mat1(+) but that mutations in mat2 appear to affect the mutational behavior of mat1. Finally, we describe a new inactive mating-type allele, mat2*, which is different from mat2(0) in that it can mutate to mat2(+).     

A CIS-Acting Mutation within the MATa Locus of SACCHAROMYCES CEREVISIAE That Prevents Efficient Homothallic Mating-Type Switching

Homothallic strains of Saccharomyces cerevisiae are able to switch from one mating-type to the other as frequently as every cell division. We have identified a cis-dominant mutation of the MATa locus, designated MATa-inc, that can be converted to MATalpha at only about 5% of the normal efficiency. In homothallic MATa-inc/mata* diploids, the MATa-inc locus switched to MATalpha in only one of 30 cases, while the mata* locus switched to MATalpha in all 30 cases. The MATa-inc mutation can be "healed" by a series of switches, first to MATalpha and then to a normal allele of MATa. These data are consistent with the "cassette" model of Hicks, Strathern and Herskowitz (1977), in which mating conversions involve the transposition of wild-type copies of a or alpha information from silent genes elsewhere in the genome. The MATa-inc mutation appears to alter a DNA sequence necessary for the replacement of MATa by MATalpha. The MATa-inc mutation has no other effect on MATa functions. In beterothallic backgrounds, the mutation has no effect on the sensitivity to alpha-factor, synthesis of a-factor, expression of barrier phenotype or ability to mate or sporulate.--The MATa-inc allele does, however, exhibit one pleiotropic effect. About 1% of homothallic MATa-inc cells become completely unable to switch mating type because of mutations at HMa, the locus proposed to carry the silent copy of alpha information.--In addition, we have isolated a less efficient allele of the HO gene.     

Mating-type control in Saccharomyces cerevisiae: isolation and characterization of mutants defective in repression by a1-alpha 2.

The alpha 2 protein, the product of the MAT alpha 2 cistron, represses various genes specific to the a mating type (alpha 2 repression), and when combined with the MATa1 gene product, it represses MAT alpha 1 and various haploid-specific genes (a1-alpha 2 repression). One target of a1-alpha 2 repression is RME1, which is a negative regulator of a/alpha-specific genes. We have isolated 13 recessive mutants whose a1-alpha 2 repression is defective but which retain alpha 2 repression in a genetic background of ho MATa HML alpha HMRa sir3 or ho MAT alpha HMRa HMRa sir3. These mutations can be divided into three different classes. One class contains a missense mutation, designated hml alpha 2-102, in the alpha 2 cistron of HML, and another class contains two mat alpha 2-202, in the MAT alpha locus. These three mutants each have an amino acid substitution of tyrosine or acid substitution of tyrosine or phenylalanine for cysteine at the 33rd codon from the translation initiation codon in the alpha 2 cistron of HML alpha or MAT alpha. The remaining 10 mutants make up the third class and form a single complementation group, having mutations designated aar1 (a1-alpha 2 repression), at a gene other than MAT, HML, HMR, RME1, or the four SIR genes. Although a diploid cell homozygous for the aarl and sir3 mutations and for the MATa, HML alpha, and HMRa alleles showed alpha mating type, it could sporulate and gave rise to asci containing four alpha mating-type spores. These facts indicate that the domain for alpha2 repression is separable from that for a1-alpha2 protein interaction or complex formation in the alpha2 protein and that an additional regulation gene, AAR1, is associated with the a1-alpha2 repression of the alpha1 cistron and haploid-specific genes.     

Efficient construction of homozygous diploid strains identifies genes required for the hyper-filamentous phenotype in Saccharomyces cerevisiae

Yeast cells undergo diploid-specific developments such as spore formation via meiosis and pseudohyphal development under certain nutrient-limited conditions. Studies on these aspects require homozygous diploid mutants, which are generally constructed by crossing strains of opposite mating-type with the same genetic mutation. So far, there has been no direct way to generate and select diploids from haploid cells. Here, we developed a method for efficient construction of homozygous diploids using a PGAL1-HO gene (galactose-inducible mating-type switch) and a PSTE18-URA3 gene (counter selection marker for diploids). Diploids are generated by transient induction of the HO endonuclease, which is followed by mating of part of the haploid population. Since the STE18 promoter is repressed in diploids, diploids carrying PSTE18-URA3 can be selected on 5-fluoroorotic acid (5-FOA) plates where the uracil prototrophic haploids cannot grow. To demonstrate that this method is useful for genetic studies, we screened suppressor mutations of the complex colony morphology, strong agar invasion and/or hyper-filamentous growth caused by lack of the Hog1 MAPK in the diploid Σ1278b strain background. Following this approach, we identified 49 suppressor mutations. Those include well-known positive regulator genes for filamentous growth signaling pathways, genes involved in mitochondrial function, DNA damage checkpoint, chromatin remodeling, and cell cycle, and also previously uncharacterized genes. Our results indicate that combinatorial use of the PGAL1-HO and PSTE18-URA3 genes is suitable to efficiently construct and select diploids and that this approach is useful for genetic studies especially when combined with large-scale screening.     

a/alpha-specific effect on the mms3 mutation on ultraviolet mutagenesis in Saccharomyces cerevisiae.

A new gene involved in error-prone repair of ultraviolet (UV) damage has been identified in Saccharomyces cerevisiae by the mms3-1 mutation. UV-induced reversion is reduced in diploids that are homozygous for mms3-1, only if they are also heterozygous (MATa/MAT alpha) at the mating type locus. The mms3-1 mutation has no effect on UV-induced reversion either in haploids or MATa/MATa or MAT alpha/MAT alpha diploids. The mutation confers sensitivity to UV and methyl methane sulfonate in both haploids and diploids. Even though mutation induction by UV is restored to wild-type levels in MATa/MATa mms3-1/mms3-1 or MAT alpha/MAT alpha mms3-1/mms3-1 diploids, such strains still retain sensitivity to the lethal effects of UV. Survival after UV irradiation in mms3-1 rad double mutant combinations indicates that mms3-1 is epistatic to rad6-1 whereas non-epistatic interactions are observed with rad3 and rad52 mutants. When present in the homozygous state in MATa/MAT alpha his1-1/his1-315 heteroallelic diploids, mms3-1 was found to lower UV-induced mitotic recombination.     

Mutations in cell division cycle genes CDC36 and CDC39 activate the Saccharomyces cerevisiae mating pheromone response pathway.

Conditional mutations in the genes CDC36 and CDC39 cause arrest in the G1 phase of the Saccharomyces cerevisiae cell cycle at the restrictive temperature. We present evidence that this arrest is a consequence of a mutational activation of the mating pheromone response. cdc36 and cdc39 mutants expressed pheromone-inducible genes in the absence of pheromone and conjugated in the absence of a mating pheromone receptor. On the other hand, cells lacking the G beta subunit or overproducing the G alpha subunit of the transducing G protein that couples the receptor to the pheromone response pathway prevented constitutive activation of the pathway in cdc36 and cdc39 mutants. These epistasis relationships imply that the CDC36 and CDC39 gene products act at the level of the transducing G protein. The CDC36 and CDC39 gene products have a role in cellular processes other than the mating pheromone response. A mating-type heterozygous diploid cell, homozygous for either the cdc36 or cdc39 mutation, does not exhibit the G1 arrest phenotype but arrests asynchronously with respect to the cell cycle. A similar asynchronous arrest was observed in cdc36 and cdc39 cells where the pheromone response pathway had been inactivated by mutations in the transducing G protein. Furthermore, cdc36 and cdc39 mutants, when grown on carbon catabolite-derepressing medium, did not arrest in G1 and did not induce pheromone-specific genes at the restrictive temperature.     

Vegetative Incompatibility and the Mating-Type Locus in the Cellular Slime Mold DICTYOSTELIUM DISCOIDEUM

The genetic basis of vegetative incompatibility in the cellular slime mold, Dictyostelium discoideum, is elucidated. Vegetatively compatible haploid strains from parasexual diploids at a frequency of between 10^-6 and 10^-5, whereas "escaped" diploids are formed between vegetatively incompatible strains at a frequency of ~10^-8. There is probably only a single vegetative incompatibility site, which appears to be located at, or closely linked to, the mating-type locus. The nature of the vegetative incompatibility is deduced from parasexual diploid formation between wild isolates and tester strains of each mating type, examination of the frequency of formation of "escaped" diploids formed between vegetatively incompatible strains, and examination of the mating type and vegetative incompatibility of haploid segregants obtained from "escaped" diploids.     

Alpha-factor structural gene mutations in Saccharomyces cerevisiae: effects on alpha-factor production and mating.

The role of alpha-factor structural genes MF alpha 1 and MF alpha 2 in alpha-factor production and mating has been investigated by the construction of mf alpha 1 and mf alpha 2 mutations that totally eliminate gene function. An mf alpha 1 mutant in which the entire coding region is deleted shows a considerable decrease in alpha-factor production and a 75% decrease in mating. Mutations in mf alpha 2 have little or no effect on alpha-factor production or mating. The mf alpha 1 mf alpha 2 double mutants are completely defective in mating and alpha-factor production. These results indicate that at least one alpha-factor structural gene product is required for mating in MAT alpha cells, that MF alpha 1 is responsible for the majority of alpha-factor production, and that MF alpha 1 and MF alpha 2 are the only active alpha-factor genes.