Mitotic versus meiotic recombination in Saccharomyces cerevisiae

Summary As part of a comparative analysis of spontaneous mitotic and meiotic recombination we have compared the mitotic and meiotic maps of the wild type and yeast hybrids homozygous for reml-l, a mitosis-specific hyper-rec mutation (Golin and Esposito, 1977; Golin, 1979). In wild type yeast strains recombination in centromere proximal intervals occurs relatively more frequently in mitosis than in meiosis. In reml-1/rem1-1 hybrids the distribution of mitotic exchange events is more similar to the distribution observed in meiosis.     

Defective karyogamy in meiotic segregants of a Candida utilis — Saccharomyces cerevisiae hybrid

Summary Meiotic segregants derived from a Candida utilis — Saccharomyces cerevisiae hybrid obtained by protoplast fusion were crossed to several standard S. cerevisiae laboratory strains. Random spore and tetrad analysis suggested that in these segregants, karyogamy is impaired and internuclear chromosome transfer occurs.     

Analysis of meiotic recombination events near a recombination hotspot in the yeast Saccharomyces cerevisiae

The region of yeast chromosome III between the HIS4 and LEU2 genes has an unusually high frequency of meiotic recombination. In order to determine the pattern of cross-over and gene conversion events, we constructed a strain with a number of heterozygous markers in this 25-kb interval. We found that very high levels of reombination are localized to regions of DNA near HIS4. In addition, analysis of the patterns of co-conversion of adjacent markers suggests that there is more than one initiation site contributing to recombination of HIS4.     

Control of replication initiation and heterochromatin formation in Saccharomyces cerevisiae by a regulator of meiotic gene expression

Heterochromatinization at the silent mating-type loci HMR and HML in Saccharomyces cerevisiae is achieved by targeting the Sir complex to these regions via a set of anchor proteins that bind to the silencers. Here, we have identified a novel heterochromatin-targeting factor for HML, the protein Sum1, a repressor of meiotic genes during vegetative growth. Sum1 bound both in vitro and in vivo to HML via a functional element within the HML-E silencer, and sum1Delta caused HML derepression. Significantly, Sum1 was also required for origin activity of HML-E, demonstrating a role of Sum1 in replication initiation. In a genome-wide search for Sum1-regulated origins, we identified a set of autonomous replicative sequences (ARS elements) that bound both the origin recognition complex and Sum1. Full initiation activity of these origins required Sum1, and their origin activity was decreased upon removal of the Sum1-binding site. Thus, Sum1 constitutes a novel global regulator of replication initiation in yeast.     

Disomic and diploid meiotic products induced in Saccharomyces cerevisiae by the salts of 27 elements

The effects of salts of 27 elements on recombination and onthe production of disomic and/or diploid spores during meiosisof Saccharomyces cerevisiae has been investigated. Be(NO₃)₂,MgSO₄, FeSO₄, CuSO₄, AgNO₃, Na₂HAsO₄ were inactive on the eventsstudied during meiotic cell division. AuCl₄, CdCl₂, C₄H₆O₄Pb,SnCl₂, K₂Cr₂O₇, RbCl induced both disomic and diploid spores.LiCI acted similarly and also affected recombination. Activityin the induction of disomic spores was shown by MnSO₄, HgCl₂and SrCl₂. CsCl, CaCl₂, Na₂MoO₄, NiCl₂, K₂PtCl⁴ increased thefrequency of diploid spores, while NaWO₄, VOSO₄, KCl, BaCl₂already increased recombination frequency. NaBiO₃ showed aneffect on meiotic recombination only. A decrease in the occurrenceof both diploid and disomic.spores was suggested by the dataobtained with CoCl₂.     

Chromosomal proteins in Saccharomyces cerevisiae

Summary Proteins were isolated from purified yeast chromatin and subjected to two-dimensional electrophoresis. The cellular and the chromosomal content of the major nonhistone proteins was measured. Two polypeptides of molecular weights 55,000 and 53,000, identified as and tubulin, and a polypeptide of molecular weight 63,000, associated with the nuclear DNA to a very high degree, account for nearly 50% of the nonhistone proteins present in chromatin. Only one tenth of the RNA polymerase subunit with the molecular weight of 23,000 was associated with nuclear DNA following chromatin purification in metrizamide gradients.     

Mitochondrial mutagenesis in Saccharomyces cerevisiae

Summary Four types of mit ^– mutations induced with manganese are found in the following relative proportions: oxi3 ^– > cob-box ^– > oxi2 ^– oxi1 ^–1. The frequences of loss of their respective mit ⁺ alleles in manganese-induced rho ^–] primary and secondary clones follow the same order. The possible interdependence between these two sets of data is discussed.     

UV-inducible proteins in Saccharomyces cerevisiae

Summary Two UV-inducible proteins have been detected in the yeast, Saccharomyces cerevisiae. The proteins have molecular weights of 78,000 Daltons and 23,000 Daltons. This induction is specific for UV-irradiation as exposure to X-rays, mitomycin C and heat shock does not result in the synthesis of the proteins. The larger (78 kD) protein is induced in various rad strains and in a ° cir° strain. Attempts are being made to isolate the genes coding for these inducible proteins.     

Stationary-phase mitophagy in respiring Saccharomyces cerevisiae

The clearance of malfunctioning mitochondria is an important housekeeping function in respiring eukaryotic cells and plays a role in physiological homeostasis as well as in the progression of late-onset diseases. This clearance is thought to occur by a specific form of autophagic degradation called mitophagy. Although the mechanism of nonspecific macroautophagy is relatively well established, the selective autophagic degradation of mitochondria has only recently begun to receive significant attention. An important step toward elucidating the mechanism by which defective mitochondria are selected and degraded is the establishment of conditions under which mitophagy is induced. This review covers our current understanding of mitophagy in the model organism Saccharomyces cerevisiae and its modes of activation, with a focus on stationary-phase mitophagy-a form of mitophagy that holds promise as a potential quality control mechanism.     

Polypeptide chain termination in Saccharomyces cerevisiae

Summary The study of translational termination in yeast has been approached largely through the identification of a range of mutations which either increase or decrease the efficiency of stop-codon recognition. Subsequent cloning of the genes encoding these factors has identified a number of proteins important for maintaining the fidelity of termination, including at least three ribosomal proteins (S5, S13, S28). Other non-ribosomal proteins have been identified by mutations which produce gross termination-accuracy defects, namely the SUP35 and SUP45 gene products which have closely-related higher eukaryote homologues (GST1-h and SUP45-h respectively) and which can complement the corresponding defective yeast proteins, implying that the yeast ribosome may be a good model for the termination apparatus existing in higher translation systems.While the yeast mitochondrial release factor has been cloned (Pel et al. 1992), the corresponding cytosolic RF has not yet been identified. It seems likely, however, that the identification of the gene encoding eRF could be achieved using a multicopy antisuppressor screen such as that employed to clone the E. coli prfA gene (Weiss et al. 1984). Identification of the yeast eRF and an investigation of its interaction with other components of the yeast translational machinery will no doubt further the definition of the translational termination process.While a large number of mutations have been isolated in which the efficiency of termination-codon recognition is impaired, it seems probable that a proportion of mutations within this class will comprise those where the accuracy of A site codon-anticodon interaction is compromised: such defects would also have an effect on termination-codon suppression, allowing mis- or non-cognate tRNAs to bind stop-condons, causing nonsense suppression. The remainder of mutatoons affecting termination fidelity should represent mutations in genes coding for components of the termination apparatus, including the eRF: these mutations reduce the efficiency of termination, allowing nonsense suppression by low-efficiency natural suppressor tRNAs. Elucidation of the mechanism of termination in yeast will require discrimination between these two classes of mutations, thus allowing definition of termination-specific gene products.     

Time-dependent mitotic recombination in Saccharomyces cerevisiae

The time-dependent appearance of prototrophic recombinants between heterologously located artificial repeats has been studied in Saccharomyces cerevisiae. While initial prototrophic colony numbers from independent cultures were highly variable, additional recombinants were found to arise daily at roughly constant rates irrespective of culture. These late-appearing recombinants could be accounted for neither by detectable growth on the selective media nor by delayed appearance of recombinants present at the time of selective plating. Significantly, at no time did the distributions of recombinants fully match those expected according to the Luria-Delbruck model and, in fact, after the first day, the distributions much more closely approximated a Poisson distribution. Prototrophic recombinants accumulated not only on the relevant selective medium, but also on media unrelated to the acquired prototrophy.     

Instability of Saccharomyces cerevisiae heterokaryons

Summary We have constructed heterokaryons of Saccharomyces cerevisiae by crossing kar1 — mutants incapable of nuclear fusion. Approximately 1% of the total zygotes from kar1 — crosses can form heterokaryotic clones. These are very small as compared to diploid colonies, and are composed mainly of a mixture of both types of heteroplasmons (cells which contain the cytoplasmic components of both parents, but the nuclear genotype of only one of them). This fact indicates that heterokaryons are unstable.This instability is already observed by pedigree analysis in the first zygotic divisions. We suggest that missegregation is the main factor in heterokaryon instability and results from an unequal nuclear transmission, which occurs when one of the mother nuclei divides and, although viable, does not migrate to the daughter bud. However, the proportion of inviable zygotes and buds found in the pedigree analysis, as well as the recovery of only one type of heteroplasmon, indicates the complete loss of one of the parental nuclei. Consequently nuclear inactivation is suggested as the second reason for heterokaryon instability.