A suppressor of mating-type locus mutations in Saccharomyces cerevisiae: evidence for and identification of cryptic mating-type loci

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.

Four genes responsible for a position effect on expression from HML and HMR in Saccharomyces cerevisiae

Mating type interconversion in Saccharomyces cerevisiae occurs by transposition of copies of the a or alpha mating type cassettes from inactive loci, HML and HMR, to an active locus, MAT. The lack of expression of the a and alpha genes at the silent loci results from repression by trans-acting regulators encoded by SIR (Silent Information Regulator) genes. In this paper we present evidence for the existence of four SIR genes. Inactivation of any of these genes leads to expression of cassettes at both HML and HMR. Unusual complementation properties are observed for a number of sir mutations. Specifically, some recessive mutations in different genes fail to complement. The correspondence between SIR1, SIR2, SIR3, SIR4 and other genes with similar roles (MAR, CMT, STE8 and STE9) is presented.

STE16, a new gene required for pheromone production by a cells of Saccharomyces cerevisiae

Genes required for mating by a and alpha cells of Saccharomyces cerevisiae (STE, "sterile," genes) encode products such as peptide pheromones, pheromone receptors, and proteins responsible for pheromone processing. a-specific STE genes are those required for mating by a cells but not by alpha cells. To identify new a-specific STE genes, we have employed a novel strategy that enabled us to determine if a ste mutant defective in mating as a is also defective in mating as alpha without the need to do crosses. This technique involved a strain (K12-14b) of genotype mata1 HML alpha HMR alpha sir3ts, which mates as a at 25 degrees and as alpha at 34 degrees. We screened over 40,000 mutagenized colonies derived from K12-14b and obtained 28 a-specific ste mutants. These strains contained mutations in three known a-specific genes--STE2, STE6 and STE14--and in a new gene, STE16. ste16 mutants are defective in the production of the pheromone, a-factor, and exhibit slow growth. Based on the distribution of a-specific ste mutants described here, we infer that we have identified most if not all nonessential genes that can give rise to a-specific mating defects.

Cell cycle control of the yeast HO gene: cis- and trans-acting regulators

In this paper, we investigate the role of a short repeated sequence (CACGA4) in the cell-cycle regulation of HO. We show that this sequence activates transcription of a heterologous gene in a cell-cycle-dependent fashion indistinguishable from that of the wild-type HO promoter. We also show that, in addition to SWI1 through SWI5, at least five other genes (SWI6 through SWI10) are required for HO transcription. These genes fit into three distinct classes with respect to their targets within the HO promoter. SWI4 and SWI6 are specifically required for CACGA4-mediated activation of transcription. SWI1, SWI2, and SWI5 are required for transcription from sequences physically separate from and independent of the CACGA4 sequences. SWI3 may be required for both. Since all the SWI genes are required for HO transcription, the HO promoter must contain at least two essential upstream activation sequences, which are affected by different trans-acting factors and are subject to different types of control.

A position effect on the expression of a tRNA gene mediated by the SIR genes in Saccharomyces cerevisiae

The SIR genes of Saccharomyces cerevisiae are responsible for the position-dependent regulation of the a and alpha mating-type genes. Previous work by others has shown that the products of the SIR genes prevent the accumulation of stable transcripts of the a and alpha genes at HML and HMR. Results of this study establish that this regulation is a region-specific effect rather than a gene-specific effect since expression of a tRNA gene placed at HMR is repressed by the products of the SIR genes.

Expression and secretion in yeast of a 400-kDa envelope glycoprotein derived from Epstein-Barr virus

The major envelope glycoprotein (gp350) of Epstein-Barr virus has been expressed and secreted in the yeast Saccharomyces cerevisiae as a 400-kDa glycoprotein. This is the first example of the secretion of such a large, heavily glycosylated heterologous protein in yeast. Since gp350 proved highly toxic to S. cerevisiae, initial cellular growth required repression of the expression of gp350. Using temperature- or galactose-inducible promoters, cells could be grown and the expression of gp350 then induced. After induction, the glycoprotein accumulated both intracellularly as well as in the culture medium. Only the most heavily glycosylated form was secreted, suggesting a role for N-linked glycans in directing secretion. The extent of O-linked glycosylation of the yeast-derived protein was similar to that of the mature viral gp350. N-linked glycosylation varied slightly depending upon culture conditions and host strain used and was more extensive than that associated with the mature viral gp350. Although there is no evidence that more than a single mRNA for the glycoprotein was expressed from the recombinant plasmid, variously sized glycoproteins accumulated in yeast at early stages after induction, probably reflecting intermediates in glycosylation. The yeast-derived glycoproteins reacted with animal and human polyclonal antibodies to gp350 as well as with a neutralizing murine monoclonal antibody to gp350, suggesting that this glycoprotein retains several epitopes of the native glycoprotein.

Map positions of yeast genes SIR1, SIR3 and SIR4

The HML and HMR loci in the yeast Saccharomyces cerevisiae each contain a complete copy of mating-type information. HML and HMR normally are transcriptionally inactive due to four unlinked genes, known as MAR or SIR or CMT. The map position of MAR1 (SIR2) has been reported previously; it is located on the left arm of chromosome IV, 27 cM from the centromere. Using conventional meiotic and mitotic mapping combined with recombinant DNA techniques, we have mapped three other SIR genes. SIR1 maps near the telomere of the right arm of chromosome XI; SIR3 (MAR2) maps to the right arm of chromosome XII, 31 cM distal to URA4; and SIR4 maps to the right arm of chromosome IV, 16 cM proximal to LYS4.

Characterization of a "silencer" in yeast: a DNA sequence with properties opposite to those of a transcriptional enhancer

The mating type of yeast is determined by the allele, either a or alpha, at the MAT locus. Two other loci, HML and HMR, contain complete copies of the alpha and a genes, respectively, which are not expressed. The four SIR gene products are required in trans for repression of the silent loci, as are cis-acting sites on either side of HML and HMR, about 1000 bp from the mating-type promoters. We demonstrate that one of these cis-acting sequences, HMRE, is able to switch off at least two nonmating-type promoters. In common with enhancers, it is able to function in either orientation, relatively independently of its position with respect to the regulated promoter, and can act on promoters 2600 bp away. However since HMRE represses, rather than enhances, transcription we have called it a "silencer" sequence.

Hybridization and Polyploidization of Saccharomyces cerevisiae Strains by Transformation-Associated Cell Fusion

Hybrid or polyploid clones of Saccharomyces cerevisiae produced by protoplast fusion were easily isolated by selecting transformants with the plasmid phenotype because the transformation was directly associated with cell fusion. When haploid cells were used as the original strain, the transformants were mostly diploids with a significant fraction of polyploids (triploids or tetraploids). Repeated transformation after curing the plasmid gave rise to clones with higher ploidy, but the frequency of cell fusion was severely reduced as ploidy increased.

Characterization of two genes required for the position-effect control of yeast mating-type genes

The mating type of haploid yeast (a or alpha) is determined by information present at the MAT locus. Identical copies of a and alpha information are present at distal loci (HMR and HML), but transcription of these copies is repressed by the action, in trans, of four unlinked genes called SIR (silent information regulator). Repression by SIR also requires, in cis, DNA sequences called E which are found to the left of HML and HMR (but not MAT) and are greater than 1 kb from the mating-type gene promoters. SIR control can act on other promoters when they are brought near the E sequence, and thus the SIR gene products act in some general manner to repress transcription. We have determined the DNA sequence of two fragments which complement mutations in the SIR2 and SIR3 genes and show that these contain the structural genes by mapping the cloned sequences onto the yeast chromosome. The SIR2 and SIR3 coding sequences were identified by constructing gene disruptions and using these mutations to replace the normal chromosomal copies. Such null mutants of both SIR2 and SIR3 are defective in the position-effect control of the silent loci but have no other detectable phenotype. We have mapped the 5' and 3' ends of the SIR2 and SIR3 mRNAs and show that their level is unaffected by mutations in any of the four known SIR complementation groups.

Role of DNA replication in the repression of silent mating type loci in yeast

A putative origin of DNA replication is associated with the DNA sequences necessary for the repression of silent mating type loci in yeast. These sequences lie about a kilobase away from the affected promoters, so the repression must act at a distance. We show here that DNA replication is required for the onset of repression.

Identification of sites required for repression of a silent mating type locus in yeast

There are three loci in the yeast Saccharomyces, each containing one of two possible genetic elements that can determine cell type. At one of these loci, MAT, this information is expressed to establish the mating type of the cell. At the other two loci, HML and HMR, this same information is phenotypically and transcriptionally silent, even though a large amount of identical sequence flanks MAT, HML and HMR coding regions. Transcriptional repression of HML and HMR requires the trans active gene products of four loci, designated variously as MAR or SIR, that are unlinked to each other or to MAT, HML or HMR. We have examined the phenotypic expression of a cloned, plasmid-borne copy of HML and of various deletion and insertion derivatives of this plasmid following their reintroduction into Mar+/Sir+ yeast strains. From these data, we have identified two sites flanking the locus, both of which are required for MAR/SIR repression of the locus. In addition, we demonstrate that each of these sites promotes autonomous replication in yeast. Abraham et al. (1984) have presented evidence demonstrating that a similar regulatory structure exists at the other silent locus, HMR. From an analysis of the sequences of these four regulatory sites, we have identified several specific sequences that may be involved in mediating repression of these loci and in promoting replication in yeast. These results are discussed in the context of potential models for the mechanism of regulation of the silent mating type loci.

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.

Regulation of mating-type information in yeast. Negative control requiring sequences both 5' and 3' to the regulated region

The genome of the yeast Saccharomyces cerevisiae contains three complete copies of the genetic information governing cell mating type. Normally, only the information in one of the copies (the MAT locus) is expressed; the other two copies (HML and HMR) are repressed and serve as donors of mating-type sequences that can be transposed to MAT in cells capable of switching mating type. We have mutagenized the silent HMR locus and have found that the repression of this locus requires two sites, one lying on each side of the mating-type sequences at HMR. The regulatory sites are positioned outside of the sequences that are included in the pair of divergent transcripts coded for by HMR, and lie about 1000 base-pairs to either side of the central promoter region of the locus. Deletion of one of the regulatory sites results phenotypically in complete loss of repression, whereas deletion of the other site gives only partial loss of control. Both of the sites are associated with an autonomous replication activity, though the relationship between this activity and the process of repression is unclear.

Transformation of protoplasted yeast cells is directly associated with cell fusion

The frequency of cell fusion during transformation of yeast protoplasts with various yeast plasmids with a chromosome replicon (YRp or YCp) or 2 mu DNA (YEp) was estimated by two methods. In one method, a mixture of protoplasts of two haploid strains with identical mating type and complementary auxotrophic nuclear markers with or without cytoplasmic markers was transformed. When the number of various phenotypic classes of transformants for the nuclear markers was analyzed by equations derived from binominal distribution theory, the frequency of nuclear fusion among the transformants was 42 to 100% in transformations with the YRp or YCp plasmids and 28 to 39% with the YEp plasmids. In another method, a haploid bearing the sir mutation, which allows a diploid (or polyploid) homozygous for the MAT (mating type) locus to sporulate by the expression of the silent mating-type loci HML and HMR, was transformed with the plasmids. Sporulation ability was found in 43 to 95% of the transformants with the YRp or YCp plasmids, and 26 to 31% of the YEp transformants. When cytoplasmic mixing was included with the nuclear fusion, 96 to 100% of the transformants were found to be cell fusants. Based upon these observations, we concluded that transformation of yeast protoplasts is directly associated with cell fusion.

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.

The new yeast genetics

Gene cloning and yeast DNA transformation techniques have greatly enhanced the power of classical yeast genetics. It is now possible to isolate any classically defined gene, to alter the yeast genome at will by replacing normal chromosomal sequences with mutated derivatives produced in vitro, and to create DNA molecules that behave as autonomous replicons or minichromosomes. These unique features of the new yeast genetics have been used to study many problems in eukaryotic molecular biology.

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.

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.

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.

A position effect in the control of transcription at yeast mating type loci

The two mating type loci MATa and MAT alpha each produce two mRNAs that are transcribed in opposite and diverging directions from central promoters. Silent copies of MATa (HMRa) and MAT alpha (HML alpha) contain identical DNA sequences throughout the transcribed region, yet are not transcribed. It is concluded that sequences to the left of HMRa (and probably HML alpha) must somehow affect transcription initiated at the centre of each locus 700 to 1,400 base pairs away. A possible mechanism for this position effect is discussed.

Homology and non-homology at the yeast mating type locus

Four mutations of the alpha mating type locus of Saccharomyces cerevisiae have been analysed to determine their relationship to the a mating type locus. Mat alpha+ recombinations are produced by mat alpha 2-/MAT but not by mat alpha 1-/MATa diploids. MAT alpha and MATa thus contain regions of homology (coding for at least part of MAT alpha 2) and regions of non-homology (coding for at least part of MAT alpha 1)-the genetic determinant for cell type is larger than the non-homologous sequence seen by DNA-DNA heteroduplexes and genetic analysis. The segment transposed in mating type interconversion includes both types of sequence.

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.

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.