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

Life cycle of the budding yeast Saccharomyces cerevisiae

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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.

Isolation of genes expressed preferentially during sporulation in the yeast Saccharomyces cerevisiae

A library of Saccharomyces cerevisiae DNA in the vector lambda Charon 28 was probed for sequences complementary to cDNA made from poly(A)+ RNA isolated from the well-sporulating yeast strain AP1 a/alpha. The RNA was isolated from cells that had been incubated 7, 9, 11, and 13 hr in sporulation medium. DNA complementary to poly(A)+ RNA from alpha/alpha(nonsporulating) AP1 was used as a control, and 46 bacteriophage that gave a stronger response with a/alpha cDNA than with alpha/alpha cDNA were obtained in a screening of three yeast genomes worth of DNA. Two of the bacteriophage appeared to contain a/alpha-specific genes, in that they hybridized to cDNA from vegetative a/alpha RNA. The rest appeared to correspond to a/alpha genes expressed preferentially during sporulation. Restriction endonuclease analysis of four of the cloned sequences revealed a single major region of transcription in each; these regions ranged in size from 2.5 to 4.0 kilobases. RNA blot analysis showed that, in three of the four cases, transcripts of two different sizes were homologous to the cloned sequence. In all four cases, the homologous transcripts appeared at about 7 hr and were decreasing in amount by 13 hr. These results provide evidence for transcriptional control of genes expressed during sporulation and for at least one group of genes that is turned on at about the time of meiosis I in sporulation.

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.

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.