Mapping of the homothallic genes, HM alpha and HMa, in Saccharomyces yeasts

The yeast mating-type switching mechanism: a memoir

It has been 33 years since I first presented results of genetic experiments that established the gene transposition model as the mechanism of mating-type switching in the budding yeast Saccharomyces cerevisiae at the Cold Spring Harbor Laboratory (CSHL) Yeast Genetics meeting in August 1977. Over two decades ago the Genetics Perspectives editors solicited a perspective on my participation in the studies that deciphered the mechanism of mating-type switching and revealed the phenomenon of gene silencing in yeast. Although flattered at the time, I thought that preparation of such an article called for a more seasoned researcher who had benefitted from seeing his contributions stand the test of time. Now realizing that our discovery of the transposition of a mutation from the HMα locus into the MAT (mating type) locus has provided the genetic evidence that established the gene transposition model, and having witnessed our conclusions confirmed by subsequent molecular studies, I decided that perhaps this is a good time to recount the chronology of events as they unfolded for me decades ago.

Extragenic suppressors of mar2(sir3) mutations in Saccharomyces cerevisiae

The silent mating-type genes (HML and HMR) of Saccharomyces cerevisiae are kept under negative transcriptional control by four trans-acting MAR (or SIR) loci. We have isolated extragenic suppressors of the mar2-1 mutation which, based on genetic complementation tests, define two additional loci involved in regulating the expression of HML and HMR. A strain with the genotype HMLa MAT alpha HMRa mar2-1 is sterile due to the simultaneous expression of a and alpha information. Two mutants exhibiting an alpha phenotype (which may result from the restoration of MAR/SIR repression) were isolated and genetically characterized. The mutations in these strains: (1) are recessive, (2) are capable of suppressing a mar2-deletion mutation, (3) are unlinked to MAT, (4) complement one another as well as the previously identified sum1-1 mutation, and (5) are not new alleles of the known MAR/SIR loci. We designate these new regulatory loci SUM2 and SUM3 (suppressor of mar). Unlike the sum1-1 mutation, suppression by sum2-1 and sum3-1 is mar2-locus specific. Both sum2-1 and sum3-1 affect the expression of a information at the HM loci. Transcript analysis shows a significant reduction in HMLa and HMRa gene transcription in mar2-1 sum2-1 and mar2-1 sum3-1 cells. Furthermore, we have found genetic evidence to suggest that mar2-1 sum2-1 cells exhibit only partial expression of silent alpha information. We conclude that the SUM2 and SUM3 gene products are required for expression of the HM loci and act downstream of the MAR2 (SIR3) gene function. Possible mechanisms for the action of the SUM gene products are discussed.

The sum1-1 mutation affects silent mating-type gene transcription in Saccharomyces cerevisiae

The silent mating-type genes (HML and HMR) of Saccharomyces cerevisiae are kept under negative transcriptional control by the trans-acting products of the four MAR/SIR loci. MAR/SIR gene mutations result in the simultaneous derepression of HML and HMR gene expression. The sum1-1 mutation was previously identified as an extragenic suppressor of mutations in MAR1 (SIR2) and MAR2 (SIR3). As assayed genetically, sum1-1 is capable of restoring repression of silent mating-type information in cells containing mar1 or mar2 null mutations. We show here that the mating-type phenotype associated with sum1-1 results from a dramatic reduction in the steady-state level of HML and HMR gene transcripts. At the same time, the sum1-1 mutation has no significant effect on the level of each of the four MAR/SIR mRNAs.

The sum1-1 mutation affects silent mating-type gene transcription in Saccharomyces cerevisiae

The silent mating-type genes (HML and HMR) of Saccharomyces cerevisiae are kept under negative transcriptional control by the trans-acting products of the four MAR/SIR loci. MAR/SIR gene mutations result in the simultaneous derepression of HML and HMR gene expression. The sum1-1 mutation was previously identified as an extragenic suppressor of mutations in MAR1 (SIR2) and MAR2 (SIR3). As assayed genetically, sum1-1 is capable of restoring repression of silent mating-type information in cells containing mar1 or mar2 null mutations. We show here that the mating-type phenotype associated with sum1-1 results from a dramatic reduction in the steady-state level of HML and HMR gene transcripts. At the same time, the sum1-1 mutation has no significant effect on the level of each of the four MAR/SIR mRNAs.

Transcriptional regulation in the yeast life cycle

The transition from haploid to diploid in homothallic yeast involves a defined sequence of events which are regulated at the level of transcription. Transcription factors encoded by SWI genes activate the HO endonuclease gene at a precise stage in the cell cycle of mother cells. The HO endonuclease initiates a transposition event which activates genes of the opposite mating type by causing them to move away from a silencer element. The activated mating type genes then regulate genes involved in cell signaling such as the mating type-specific pheromones and their receptors. Since HO is only activated in one of the sister cells after division (the mother), adjacent cells of opposite mating type are generated which respond to each others' secreted pheromones by inducing genes involved in conjugation. This leads to the formation of a diploid in which many of the genes involved in mating and mating-type switching become repressed due to the heterozygosity of the mating-type locus. This article summarizes what is known about these transcriptional controls and discusses possible parallels in higher eukaryotes.

Resolution of recombination intermediates generated during yeast mating type switching

Interchromosomal gene conversion between alleles has been shown in yeast frequently to be associated with the recombination of flanking genetic markers. Although this also holds true for gene conversion between two alleles of the yeast mating-type (MAT) locus, initiated by the homothallic switching system, we find no evidence that crossing-over ever accompanies gene conversion between the non-allelic HMR and MAT genes when initiated by this same homothallic switching system.

Mating type control in Saccharomyces cerevisiae: a frameshift mutation at the common DNA sequence, X, of the HML alpha locus

A mutation defective in the homothallic switching of mating type alleles, designated hml alpha-2, has previously been characterized. The mutation occurred in a cell having the HO MATa HML alpha HMRa genotype, and the mutant culture consisted of ca. 10% a mating type cells, 90% nonmater cells of haploid cell size, and 0.1% sporogenous diploid cells. Genetic analyses revealed that nonmater haploid cells have a defect in the alpha 2 cistron at the MAT locus. This defect was probably caused by transposition of a cassette originating from the hml alpha-2 allele by the process of the homothallic mating type switch. That the MAT locus of the nonmater cells is occupied by a DNA fragment indistinguishable from the Y alpha sequence in electrophoretic mobility was demonstrated by Southern hybridization of the EcoRI-HindIII fragment encoding the MAT locus with a cloned HML alpha gene as the probe. The hml alpha-2 mutation was revealed to be a one-base-pair deletion at the ninth base pair in the X region from the X and Y boundary of the HML locus. This mutation gave rise to a shift in the open reading frame of the alpha 2 cistron. A molecular mechanism for the mating type switch associated with the occurrence of sporogenous diploid cells in the mutant culture is discussed.

A site-specific endonuclease essential for mating-type switching in Saccharomyces cerevisiae

We have detected two site-specific endonucleases in strains of Saccharomyces cerevisiae. One endonuclease, which we call YZ endo, is present only in yeast strains that are undergoing mating-type interconversion. The site at which YZ endo cleaves corresponds to the in vivo double-strand break occurring at the mating-type locus in yeast undergoing mating-type interconversion. YZ endo generates a site-specific double-strand break having 4-base 3' extensions terminating in 3' hydroxyl groups. The site of cleavage occurs in the Z1 region near the YZ junction of the mating-type locus. Mutant mating-type loci known to decrease the frequency of mating-type interconversion are correspondingly poor substrates for YZ endo in vitro. In vitro analysis of a number of such altered recognition sites has delimited the sequences required for cleavage. The molecular genetics of mating-type interconversion is discussed in the context of this endonucleolytic activity. The second endonuclease, which we refer to as Sce II, is present in all strains of S. cerevisiae we have examined. The cleavage site of Sce II has been determined and proves to be unrelated to the cleavage site of YZ endo.

A mutation allowing expression of normally silent a mating-type information in Saccharomyces cerevisiae

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.

Efficient production of a ring derivative of chromosome III by the mating-type switching mechanism in Saccharomyces cerevisiae

The mating-type switches in the yeast Saccharomyces cerevisiae occur by unidirectional transposition of replicas of unexpressed genetic information, residing at HML or HMR, into the mating-type locus (MAT). The source loci, HML and HMR, remain unchanged. Interestingly, when the HM cassettes are expressed, as in marl strains, the HML and HMR cassettes can also efficiently switch, apparently by obtaining genetic information from either of the other two cassettes (Klar et al., Cell 25:517-524, 1981). We have isolated a novel chromosome III rearrangement in heterothallic (marl ho) strains, which is also produced efficiently in marl HO cells, presumably the consequence of a recombination event between HML and HMR. The fusion results in the loss of sequences which are located distal to HML and to HMR and produces a ring derivative of chromosome III. Cells containing such a ring chromosome are viable as haploids; apparently, no essential loci are located distal to the HM loci. The fusion cassette behaves as a standard HM locus with respect to both regulation by the MAR/SIR control and its role in switching MAT.

Homothallic switching of yeast mating type cassettes is initiated by a double-stranded cut in the MAT locus

A double-stranded DNA cut has been observed in the mating type (MAT) locus of the yeast Saccharomyces cerevisiae in cultures undergoing homothallic cassette switching. Cutting is observed in exponentially growing cells of genotype HO HML alpha MAT alpha HMR alpha or HO HMLa MATa HMRa, which switch continuously, but not in a/alpha HO/HO diploid strains, in which homothallic switching is known to be shut off. Stationary phase cultures do not exhibit the cut. Although this site-specific cut occurs in a sequence (Z1) common to the silent HML and HMR cassettes and to MAT, only the Z1 sequence at the MAT locus is cut. The cut at MAT occurs in the absence of the HML and HMR donor cassettes, suggesting that cutting initiates the switching process. An assay for switching on hybrid plasmids containing mata- cassettes has been devised, and deletion mapping has shown that the cut site is required for efficient switching. Thus a double-stranded cut at the MAT locus appears to initiate cassette transposition-substitution and defines MAT as the recipient in this process.

Transposition of yeast mating type genes from two translocations of the left arm of chromosome III

In the yeast Saccharomyces cerevisiae, the HIS4C gene lies on the left arm of chromosome III. We analyzed two chromosomal rearrangements that have HIS4C translocated either to chromosome XII or to a new translocation chromosome. Using the cmt mutation that allows expression of the normally silent copies of mating type genes, we found that both of these translocations also carried HML alpha, more than 30 map units distal to HIS4C which normally lies on chromosome III. In the case of the translocation chromosome (designated T3), we also found an exchange event between HML alpha on the translocation chromosome and HMLa on chromosome III. In diploids containing two T3 chromosomes (one carrying HML alpha and the carrying HMLa), we found that HML was 32 centimorgans from HIS4C, which was 10 centimorgans from an unknown centromere. In homothallic strains carrying HMLa MATa HMRa on chromosome III, switching from MATa to MAT alpha could occur by using the HML alpha on the translocation as the sole donor of alpha information. Transposition from HML alpha on chromosome T3 was about 20 to 40% as efficient as transposition from intact chromosome III. In contrast, transposition from the HML alpha inserted into chromosome XII was reduced about 100-fold. This reduced efficiency did not appear to be caused by an alteration in the sequences immediately surrounding HML alpha in the translocation. The translocated HML alpha sequence was located in the same size (29-kilobase) SalI fragment as was found in chromosome III, and the same EcoRI, HindIII, and BglII restriction sites were also found. Furthermore, HML alpha was still under the control of the CMT gene, which maintains HML as a silent copy of mating type information. These results suggested that the position of the HML alpha sequence plays an important role in the efficiency of mating type switching.

Changes in production of the mating-type-specific glycoproteins, agglutination substances in association with mating type interconversion in homothallic strains of the yeast, Saccharomyces cerevisiae

Sexual activity in homothallic strains of Saccharomyces cerevisiae was investigated. We succeeded in culturing homothallic haploid cells without conjugation, by lowering the pH value of the culture medium. In spore cultures of a homothallic strain both a and alpha pheromones were detected. Agglutination substance of a and alpha mating types were detected in homothallic haploid cells from spore cultures in early logarithmic phase regardless of mating type information at the HML and HMR loci, but either a or alpha agglutination substance was detected predominantly in homothallic haploid cells from spore culture in late logarithmic phase, depending on mating type information at the HML and HMR loci.

Directionality of yeast mating-type interconversion

The mating-type a and alpha alleles of the yeast Saccharomyces cerevisiae interconvert by a transposition-substitution reaction where replicas of the silent mating loci, at HML and HMR, are transmitted to the expressed mating-type locus (MAT). HML is on the left arm and HMR on the right arm, while MAT is in the middle of chromosome III. Cells with the genotype HML alpha HMRa switch mating type efficiently at a frequency of about 86%. Since well over 50% of the cells switch, it is thought that switches do not occur randomly, but are directed to occur to the opposite mating-type allele. In contrast, we report that strains possessing the reverse HMLa HMR alpha arrangement switch (phenotype) inefficiently at a maximum of about 6%. The basis for this apparent reduced frequency of switching is that these strains preferentially yield futile homologous MAT locus switches--that is, MATa to MATa and MAT alpha to MAT alpha--and consequently, most of these events are undetected. We used genetically marked HM loci to demonstrate that alpha cells preferentially choose HMR as donor and a cells preferentially choose HML as donor, irrespective of the genetic content of the silent loci. Because of this feature, HML alpha HMRa strains generate predominantly heterologous while HMLa HMR alpha strains produce predominantly homologous MAT switches. The control for directionality of switching therefore is not at the level of transposing heterologous mating-type information, but only at the level of choosing HML versus HMR as the donor. In strains where the preferred donor locus is deleted, the inefficient donor becomes capable of donating efficiently. Thus the preference seems to be mediated by competition between the HM loci for donating information to MAT.

A position-effect control for gene transposition: state of expression of yeast mating-type genes affects their ability to switch

Mating-type switches of the yeast Saccharomyces cerevisiae occur by unidirectional transposition of copies of unexpressed mating-type genetic information, residing at HML and HMR loci, into the expressed MAT locus. The HML and HMR loci remain unchanged. In contrast, in appropriate strains where the silent loci are also allowed to express, for example in mar mutants, efficient switches of HML and HMR are shown to occur at rates equivalent to those observed for MAT. Thus the position-effect control on the direction of transposition is affected by the state of expression of the locus under study the expressed loci switch regardless of their location.

Gene conversion between duplicated genetic elements in yeast

The mitotic recombination behaviour of a duplication of the his4 region on chromosome III in the yeast Saccharomyces cerevisiae was studied. The major recombination event between the duplicated segments is gene conversion unassociated with reciprocal recombination. The rad52-1 mutation preferentially decreases mitotic gene conversion. These results suggest that mitotic gene conversion may occur by a different pathway from that occurring in meiosis. This mitotic gene conversion may be important in yeast mating type interconversion and the maintenance of sequence homogeneity in families of repeated eukaryotic genes.

Regulation of transcription in expressed and unexpressed mating type cassettes of yeast

The genes that control the a, alpha and a/alpha cell types in Saccharomyces are carried on transposable elements known as a and alpha cassettes which reside at three different chromosomal loci. Examination of the transcripts by R-looping and filter hybridization indicates that each cassette is capable of producing two divergent transcripts. Cassettes at the MAT locus are transcribed constitutively. Transcription of cassettes at HML and HMR is prevented by trans-acting negative regulators.

Evidence for a physical interaction between the transposed and the substituted sequences during mating type gene transposition in yeast

Mating type switches in the yeast Saccharomyces cerevisiae occur by transposition of a replica of the "source" unexpressed loci HML and HMR to the mating type locus (MAT). The incoming information replaces previously expressed DNA, resulting in an interconversion of MAT alleles. A strain of genotype HML alpha/HML alpha MAT alpha/mata-missense HMR alpha/hmra-nonsense HO/ho generates cells with the genotype HML alpha/HML alpha MAT alpha/MAT a HMR alpha/hmra-nonsense HO/ho; that is, wild-type MATa+ recombinants are produced efficiently by a strain in which the incoming a information and the resident mata allele bear different mutations. Production of the wild-type MATa recombinants requires the homothallism (switching) function, and the incoming a information and the resident mata allele must bear different mutations. This result is consistent with the formation of a heteroduplex between the incoming and the outgoing DNA at MAT. Thus a process of unidirectional gene conversion as a mechanism for mating type gene transposition is favored. A molecular model based on a single-strand transfer is proposed. Results also favor the idea that the direction of switching is controlled by cell's mating phenotype rather than by the genetic content of MAT.

The trans action of HMRa in mating type interconversion

HML and HMR are the sites of cryptic mating type genes in the yeast Saccharomyces cerevisiae. In the presence of the HO gene, the information from HML or HMR (an a or alpha cassette) is transferred to the mating type locus (MAT). HML, HMR, and MAT are located on chromosome III, yet are widely separated. Similarly, in other yeasts, at least some of the genes involved in mating typing interconversion are linked to the mating type locus. We demonstrate here that a cassette donor (HMR) and the cassette target (MAT) need not be physically linked for successful mating type interconversion. In particular, we show that HMRa on one chromosome can donate an a cassette to the mating type locus on a homologous chromosome III.

Functional equivalence and co-dominance of homothallic genes HM alpha/hm alpha and HMa/hma in Saccharomyces yeasts

The specificity of mating type in Saccharomyces yeasts is controlled by a pair of alleles, a and alpha, on chromosome III. They are mutually interconverted by the function of three kinds of homothallic genes, each consisting of a single pair of alleles, HO/ho, HM alpha/hma alpha and HMa/hma. For the a to alpha conversion, HO HM alpha, HMa, HO hm alpha HMa and HO hma alpha hma genotypes are effective; whereas the alpha to a conversion occurs in HO HM alpha HMa, HO HM alpha hma and HO hm alpha hma cells. To explain these observations, NAUMOV and TOLSTORUKOV (1973) and HARASHIMA, NOGI and OSHIMA (1974) suggested that hma and HM alpha are functionally equivalent and effective for the alpha to a conversion in combination with HO; whereas, hm alpha and HMa are functionally equivalent and effective for the a to alpha conversion with the function of HO. To test this idea and to compare it with two other possible mechanisms, some of the tetrad segregants from four kinds of a/a/alpha/alpha tetraploids homozygous for the HO allele and for one of the HM alpha/hm alpha and HMa/hma loci, while heterozygous for the other one with +/+/-/- configuration, were investigated with respect to their thallism by self-sporulation. Results indicated the functional equivalence of both the HM alpha and hma alleles and the hm alpha and HMa alleles in mating-type conversion, and the co-dominance of the alleles of each locus. From the findings and other data, we agree with the revision of the nomenclature of the HM alpha/hm alpha and HMa/hma genes to HMRa/HMR alpha and HML alpha/HMLa, respectively.

Interconversion of yeast cell types by transposable genes

The a and alpha cell types of budding yeast saccharomyces cerevisiae are controlled by alternate alleles of the mating-type locus (MAT), MATa and mat alpha. The cell types can be interconverted by switching alleles of MAT. The loci HMRa and HML alpha, which are loosely linked to MAT, are involved in mating-type switching. Experimental evidence for their role in MAT interconversion is presented. As a result of switching, the homothallic and heterothallic strains containing the amber and ochre mutations within the HMRa locus yield corresponding amber and ochre mutant mata loci. Similarly, the hml alpha mutant strain generates mat alpha mutant alleles. That is, specific mutations from HMRa and HML alpha are transmitted to MAT. A replica of the mating-type coding information originating from these loci is transposed to MAT, where it replaces the existing information. Furthermore, "Hawthorne deletions" in strains containing hmra-amber/ochre result in production of mata-amber/ochre alleles. Therefore, genetic information for MATa resides at HMRa. The switches occur in a defined set of clonally related cells. Thus, the efficient interconversion of yeast cell types is mediated by an unidirectional transfer of genetic information between nonallelic sites in a nonrandom and programmed fashion. The results are inconsistent with the "flip-flop" models, but satisfy a key prediction of the general controlling element and the specific cassette models proposed for mating-type interchange.

Duplicated genes producing transposable controlling elements for the mating-type differentiation in Saccharomyces cerevisiae

Mutation of the two homothallic genes, HML alpha/HMLa and HMRa/HMR alpha, in homothallic strains of Saccharomyces cerevisiae was studied. Of 11 mutants of the HML alpha gene, eight were due to a phenotypic mutation from HML alpha to HMLa, i.e., a mutation causing a change in function of the original HML allele to that of the other HML allele (functional mutation), and three were due to a defective mutation at the HML alpha gene, i.e., a mutation causing a nonfunctional allele (nonfunctional mutation). All 14 mutants of the HMRa gene, on the other hand, were due to a phenotypic mutation from HMRa to HMR alpha i.e., a functional mutation. Phenotypic reverse mutations, i.e., HMLa to HML alpha and HMR alpha to HMRa, were also observed in the cultivation of EMS (ethyl methanesulfonate) treated spores having the HO HMR alpha HMLa genotype. Mutation from heterothallic cells to homothallism was observed in a nonfunctional mutant of the HML alpha gene, by mutagenesis with EMS, but not in the functional mutants of the HML alpha and HMRa genes or in the authentic strains having the alpha HO HMR alpha HML alpha (alpha Hp) and a HO HMRa HMLa (a Hq) genotypes. These observations suggest that the functional mutation is not caused by the direct mutation from a homothallic allele to the opposite, but by replacement of a transposable genic element produced from a homothallic locus with a region of a different homothallic locus. These observations also support the controlling-element model and the cassette model, which have been proposed to explain the mating-type differentiation by the homothallic genes.

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.

Transposable mating type genes in Saccharomyces cerevisiae

A functional copy of the alpha mating type gene of Saccharomyces cerevisiae has been cloned by transformation in yeast. Using the Southern Blotting procedure it has been shown that three distinct genetic loci implicated in mating type interconversion (HML, HMR and MAT) contain sequences homologous to the clone fragment. The restriction fragment associated with each locus exhibits a characteristic size which can be correlated with the mating type allele present at that locus. The characteristic size difference between the a and alpha genetic elements made it possible to demonstrate that the homothallic interconversion of mating types in this yeast occurs by DNA rearrangement as proposed in the 'cassette hypothesis'.

Isolation of a circular derivative of yeast chromosome III: implications for the mechanism of mating type interconversion

We describe genetic and physical characterization of rearrangements of chromosome III which result in changes of cell type in S. cerevisiae. Two types of rearrangements were obtained as rare events which caused a change at the locus controlling cell type, MAT, associated with a recessive lethal mutation, in one case from MATalpha to MATa-lethal, and in the other case from MATa to MATalpha-lethal. The MATa-lethal mutation is a deletion on the right arm of chromosome III, which we demonstrate extends to (or near) HMalpha. We suggest this deletion removes MATalpha and activates cryptic MATa information stored in HMalpha as proposed in the cassette model of mating type interconversion. The MATalpha-lethal mutation is the result of the formation of a circular chromosome III, which we interpret to remove MATa and activate the cryptic MATalpha information stored at HMa. Strains carrying the MATalpha-lethal chromosome contain a circular chromosome of length 62.6 plus or minus 5.7 mum, which is absent in related strains. This chromosome was confirmed to be chromosome III by hybridization of specific yeast DNA fragments to supercoiled DNA obtained from MATalpha-lethal strains. The isolation of a large circular derivative of chromosome III allows correlation of genetic and physical distance based on large distances-1 centimorgan corresponds to approximately 2700 base pairs.

Activation of mating type genes by transposition in Saccharomyces cerevisiae

Yeast Saccharomyces cerevisiae may express an a or alpha mating type. These cells types may be interconverted as a consequence of heritable genetic alteractions at the mating type locus (MAT). According to the more general controlling element model [Oshima, U. & Takano, I. (1971) Genetics 67, 327--335] and the specific cassette model [Hicks, J., Strathern, J. & Herskowitz, I. (1977) in DNA Insertion Elements, Plasmids and Episomes, eds. Bukhari, A. I., Shapiro, J.A. & Adhya, S. L.(Cold Spring Harbor Laboratory, Cold Spring Harbor, NY), pp. 457--462], the regulatory information required for switching the MAT locus exists at two other loosely linked loci, HMa and HMalpha. Specifically, the HMa and HMalpha loci are proposed to carry silent alpha and silent a genes, respectively. According to these models, switching occurs when a replica of a silent gene replaces the resident information at the mating type locus and is thereby expressed. These models predict that mutations at the silent ("storage") loci would generate defective MAT loci subsequent to the switching process. Therefore, the behavior of HMalpha mutants during the mating type interconversion was investigated. The results demonstrate that defective MATa alleles are generated by switching the MATalpha locus in HMalpha mutants. Thus, the genetic information from HMalpha is transposed to the mating type locus. These results provide genetic evidence in support of these models.

Gene conversion of the mating-type locus in Saccharomyces cerevisiae

Tetrad analysis of MATa/MAT alpha diploids of Saccharomyces cerevisiae generally yields 2 MATa:2MAT alpha meiotic products. About 1 to 1.8% of the tetrads yield aberrant segregations for this marker. Described here are experiments that determine whether the aberrant meiotic segregations at the mating-type locus are ascribable to gene conversions or to MAT switches, that is, to mating-type interconversions. Diploid strains incapable of switching MATa to MAT alpha, or the converse, nevertheless display changes of MATa to MAT alpha, or the reverse. These events must be attributed to gene conversion. Further, we suggest that MATa and MAT alpha alleles may represent nonhomologous sequences of DNA since they fail to display postmeiotic segregations.

Frequencies of twelve ascus-types and arrangement of three genes from tetrad data

Equations expressing the theoretical frequencies of twelve ascus-types in the tetrad analysis of a triply heterozygous diploid are described. Using these equations, a mapping procedure for a gene X, is proposed. The procedure requires that two genes, X and Y, of the same phenotype be heterozygous and that the map position of Y be known, and that another standard gene, Z, show an independent phenotype from X and Y. This procedure does not require the laborious allelism test of the segregants to determine the allelic 2:2 segregation in tetrads for the X and Y genes, which is indispensable for mapping by the conventional procedure. The exact placement of the X gene on a chromosome is possible by the chi2 minimization procedure in comparison with the expected frequencies of the six ascus-types or four spore-types deduced from the twelve expected ascus-types to give the optimal fit with the observed data.