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G Simchen

Publications and source records attributed to G Simchen.

At least 55 records · Page 3Linked to original sources
PubMed1984

Cloning and mapping of the RAD50 gene of Saccharomyces cerevisiae.

The RAD50 gene was cloned as a 4.8 kb fragment in the 2 mu derived plasmid pFL1. The gene resides in a 3.9 kb segment that was subcloned into the plasmid YRp7. The cloned gene complements the deficiency caused by the rad50-1 mutation with respect to gamma-rays, MMS resistance and UV-induced mitotic recombination. Restoration of the Rad+ phenotype occurs when the cloned gene is on a freely replicating multiple-copy plasmid or in the integrated form. Mapping of the cloned gene following integration of the 2 mu plasmid, and of the subclone in plasmid YRp7, showed it to be located on the left arm of chromosome XIV. Tetrad analysis of various crosses involving two different strains carrying rad50-1 showed the mutation to map next to pet2 on chromosome XIV, and not on the right arm of chromosome IV, as previously published.

Cloning, Molecular↗
PubMed1984

Ty-mediated gene expression of the LYS2 and HIS4 genes of Saccharomyces cerevisiae is controlled by the same SPT genes.

Five Ty insertion mutations were isolated at the LYS2 locus of Saccharomyces cerevisiae. Genetic and physical analyses show that four Ty insertions are in the 5' noncoding region of LYS2 and one is within the structural gene. Three of these Ty elements have been cloned and characterized. The Ty mutations differ from each other in restriction pattern, phenotypic effects on LYS2, reversion frequency, and the nature of reversion events. Spt2 and spt3 mutations, known to suppress Ty insertions and their solo delta derivatives at HIS4, can also suppress at least one of the Ty insertions (Ty61) at LYS2 and can also suppress the Lys- phenotype of a solo delta derivative of another Ty insertion (Ty128) at LYS2. These results demonstrate that spt mutations can suppress Ty and delta mutations at both HIS4 and LYS2, suggesting that they are general for their effects on Ty and delta elements.

Cloning, Molecular↗
PubMed1983

The timing of the S phase and other nuclear events in yeast meiosis.

The length of the premeiotic S phase in individual cells of a homothallic strain of Saccharomyces cerevisiae was determined by pulse-labelling synchronously sporulating cultures with [3H]adenine and following changes in the frequency of cells in S by DNA-specific whole cell autoradiography. The average S phase was found to be at least 2-3 times as long as in mitotic diploids, and it was concluded that activation of replication origins in meiosis is considerably staggered. The timing of S in relation to other meiotic milestones was established by counting different nuclear morphologies visualised by DAPI-fluorescent staining. This allowed tentative identification of pachytene nuclei as well as first and second nuclear divisions.

Autoradiography↗
PubMed1981

Elevated recombination and pairing structures during meiotic arrest in yeast of the nuclear division mutant cdc5.

A diploid strain of yeast, homozygous for the mutation cdc5-1, undergoes a normal meiosis at 25 degrees C. At the nonpermissive temperature of 34 degrees C, meiosis is arrested at the first meiotic division, after premeiotic DNA replication and recombination commitment have taken place. Haploidisation commitment does not occur at 34 degrees C. Electron microscopy reveals that synaptons (synaptonemal complexes) are formed and the stage of arrest is characterised by a prevalence of "modified synaptons", which consist of paired lateral elements lacking the central elements. Prolonged incubation at this stage of arrest results in unusually high recombination levels, perhaps related to the synaptonal structures observed. Temperature shift-up experiments (transfers of cell from 25 degrees C to 34 degrees C at various times during meiosis) reveal that the CDC5 function is required for both the first and the second divisions of meiosis.

Hot Temperature↗
PubMed1979

Morphogenesis of the synapton during yeast meiosis.

The formation of the synapton (synaptonemal complex) was followed by an electron microscopic examination of large samples of Saccharomyces cerevisiae cells at various stages of meiosis. Three temperature-sensitive mutants were used, cdc4, cdc5 and cdc7, which undergo a slow but normal meiosis at 25 degrees C. At the restrictive temperature of 34 degrees C, cdc4 and cdc5 arrest at an advanced enough stage of meiosis to allow the study of synapton morphogenesis. Based on the frequencies of nuclear structures, we describe the formation of the central region and central elements of the synapton in the dense body, which may be part of the nucleolus. This process occurs during early meiotic stages, concomittantly with recombination commitment and premeiotic DNA replication. Mature synaptons usually appear after premeiotic S, at the pachytene stage, and later disappear. A possible intermediate stage in this disappearance is found in arrested cdc5 cells, which contain paired lateral elements without central elements. Following the frequencies of spindle plaque configurations, we conclude that the plaques in meiosis duplicate once at the beginning of the main DNA replication, as is also observed prior to mitosis. In contrast to mitotic cells, however, meiotic plaques remain duplicated for a long period, until the synaptons disappear, and only then separate from each other to form a spindle. During late stages of the first meiotic division, the outer plates of the spindle plaques thicken, to duplicate later and give the second division spindles. The characteristically thick outer plate may have a role in the formations of the ascopore wall.

Meiosis↗
PubMed1978

Meiotic recombination and DNA synthesis in a new cell cycle mutant of Saccharomyces cerevisiae.

Vegetative cells carrying the new temperature-sensitive mutation cdc40 arrest at the restrictive temperature with a medial nuclear division phenotype. DNA replication is observed under these conditions, but most cells remain sensitive to hydroxyurea and do not complete the ongoing cell cycle if the drug is present during release from the temperature block. It is suggested that the cdc40 lesion affects an essential function in DNA synthesis. Normal meiosis is observed at the permissive temperature in cdc40 homozygotes. At the restrictive temperature, a full round of premeiotic DNA replication is observed, but neither commitment to recombination nor later meiotic events occur. Meiotic cells that are already committed to the recombination process at the permissive temperature do not complete it if transferred to the restrictive temperature before recombination is realized. These temperature shift-up experiments demonstrate that the CDC40 function is required for the completion of recombination events, as well as for the earlier stage of recombination commitment. Temperature shift-down experiments with cdc40 homozygotes suggest that meiotic segregation depends on the final events of recombination rather than on commitment to recombination.

Cell Cycle↗
PubMed1978

Cell cycle mutants.

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Aspergillus nidulans↗
PubMed1977

Effects of the mitotic cell-cycle mutation cdc4 on yeast meiosis.

The mitotic cell-cycle mutation cdc4 has been reported to block the initiation of nuclear DNA replication and the separation of spindle plaques after their replication. Meiosis in cdc4/cdc4 diploids is normal at the permissive temperature (25 degrees) and is arrested at the first division (one-nucleus stage) at the restrictive temperature (34 degrees or 36 degrees). Arrested cells at 34 degrees show a high degree of commitment to recombination (at least 50% of the controls) but no haploidization, while cells arrested at 36 degrees are not committed to recombination. Meiotic cells arrested at 34 degrees show a delayed and reduced synthesis of DNA (at most 40% of the control), at least half of which is probably mitochondrial. It is suggested that recombination commitment does not depend on the completion of nuclear premeiotic DNA replication in sporulation medium.--Transfer of cdc4/cdc4 cells to the restrictive temperature at the onset of sporulation produces a uniform phenotype of arrest at a 1-nucleus morphology. On the other hand, shifts of the meiotic cells to the restrictive temperature at later times produce two additional phenotypes of arrest, thus suggesting that the function of cdc4 is required at several points in meiosis (at least at three different times).

DNA Replication↗
PubMed1976

DNA Degradation and reduced recombination following UV irradiation during meiosis in yeast (Saccharomyces cerevisiae).

Irradiation of meiotic yeast cells with moderate doses of ultraviolet irradiation (1,600 erg/mm2) leads to the arrest of premeiotic DNA synthesis, massive (5-40%) DNA degradation, and a 40-50% loss of cell viability. In contrast, such doses of UV irradiation had a minor effect on viability (15-20% loss) of logarithmically growing cells, and no comparable DNA degradation was observed in irradiated synchronized vegetative cells. Meiotic recombination is also affected by UV irradiation. When administered at a stage comparable to meiotic prophase, low doses of irradiation result in a reduction in recombination frequency without significantly affecting cell viability.

DNA↗
PubMed1976

Recombination and hydroxyurea inhibition of DNA synthesis in yeast meiosis.

Hydroxyurea (HU) inhibits the premeiotic DNA replication and the meiotic events that followed, namely readiness, recombination commitment, haploidisation, sporulation commitment and ascus formation. Short incubations with HU (2-4 hrs) during the premeiotic replication (i.e. starting between 3 and 6.5 hrs in sporulation medium) allow the resumption of the replication at a normal rate following the removal of the drug. The other meiotic events are similarly delayed by the approximate length of the treatment. In these experiments, intragenic recombination in ade2 reached a higher level than in the controls (x 1.3-2.0 in one pair of heteroalleles and x 3.0-4.0 in another pair). The recombination response to short HU treatments was not observed for a pair of heteroalleles in ade2 that normally shows a high level of meiotic recombination (750 per 10(6) cells), nor was the response observed in a pair of heteroalleles in lys2. HU treatments have almost no effect on sporulating cells from 8 hrs onwards. At 7-7.5 hrs the meiotic cells are very sensitive to the drug and even short treatments cause cell death and massive DNA degradation.

DNA Replication↗
PubMed1976

Regulation of mating and meiosis in yeast by the mating-type region.

A supposed sporulation-deficient mutation of Saccharomyces cerevisiae is found to affect mating in haploids and in diploids, and to be inseparable from the mating-type locus by recombination. The mutation is regarded as a defective a allele and is designated a*. This is confirmed by its dominance relations in diploids, triploids, and tetraploids. Tetrad analysis of tetraploids and of their sporulating diploid progeny suggests the existence of an additional locus, RME, which regulates sporulation in yeast strains that can mate. Thus the recessive homozygous constitution rme/rm- enables the diploids a*/alpha, a/a*, and alpha/alpha to go through meiosis. Haploids carrying rme show apparent premeiotic DNA replication in sporulation conditions. This new regulatory locus is linked to the centromere of the mating-type chromosome, and its two alleles, rme and RME, are found among standard laboratory strains.

Alleles↗
PubMed1974

Are mitotic functions required in meiosis?

Sporulation of diploid yeasts (Saccharomyces cerevisiae), homozygous or heterozygous for temperature-sensitive mitotic cell-cycle mutations, was examined at the restrictive and permissive temperatures. Twenty genes, represented by 32 heterozygotes and 60 homozygotes, were divided into three groups, showing (i) normal sporulation, (ii) no sporulation at the restrictive temperature but normal sporulation at the permissive temperature, (iii) no sporulation at both temperatures. Group (i) as well as several other strains were tested for their meiotic behavior with regard to intragenic recombination and haploidization. The conclusion reached was that all the mitotic nuclear-division and DNA-synthesis functions were required in meiosis. The only cell-division mutations not to affect meiosis were in three cytokinesis loci and in one budemergence locus.

Chromosome Mapping↗