Characterization of ICR-170-induced mutations in Schizosaccharomyces pombe

Schizosaccharomyces versatilis represents a distinct evolutionary lineage of fission yeast

The fission yeast species Schizosaccharomyces japonicus is currently divided into two varieties-S. japonicus var. japonicus and S. japonicus var. versatilis. Here we examine the var. versatilis isolate CBS5679. The CBS5679 genome shows 88% identity to the reference genome of S. japonicus var. japonicus at the coding sequence level, with phylogenetic analyses suggesting that it has split from the S. japonicus lineage 25 million years ago. The CBS5679 genome contains a reciprocal translocation between chromosomes 1 and 2, together with several large inversions. The products of genes linked to the major translocation are associated with 'metabolism' and 'cellular assembly' ontology terms. We further show that CBS5679 does not generate viable progeny with the reference strain of S. japonicus. Although CBS5679 shares closer similarity to the 'type' strain of var. versatilis as compared to S. japonicus, it is not identical to the type strain, suggesting population structure within var. versatilis. We recommend that the taxonomic status of S. japonicus var. versatilis is raised, with it being treated as a separate species, Schizosaccharomyces versatilis.

Introduction to Fission Yeast as a Model System

Here, we briefly outline the history of fission yeast, its life cycle, and aspects of its biology that make it a useful model organism for studying problems of eukaryotic molecular and cell biology.

Live Imaging of Chromosome Segregation during Meiosis in the Fission Yeast Schizosaccharomyces pombe

This protocol describes the live observation of chromosome segregation during fission yeast meiosis. To visualize one chromosome of interest, the lac operator (lacO array) is integrated at its centromere-proximal locus, and the lac repressor (lacI)-GFP fusion protein is expressed in a haploid strain. This haploid strain, in which mCherry-tagged tubulin is also expressed exogenously to monitor meiotic progression, is crossed with a nonlabeled haploid strain to induce meiosis. GFP and mCherry signals in resulting zygotes are observed by a fluorescent microscopy during the progression of meiosis.

Fission Yeast SCYL1/2 Homologue Ppk32: A Novel Regulator of TOR Signalling That Governs Survival during Brefeldin A Induced Stress to Protein Trafficking

Target of Rapamycin (TOR) signalling allows eukaryotic cells to adjust cell growth in response to changes in their nutritional and environmental context. The two distinct TOR complexes (TORC1/2) localise to the cell’s internal membrane compartments; the endoplasmic reticulum (ER), Golgi apparatus and lysosomes/vacuoles. Here, we show that Ppk32, a SCYL family pseudo-kinase, is a novel regulator of TOR signalling. The absence of ppk32 expression confers resistance to TOR inhibition. Ppk32 inhibition of TORC1 is critical for cell survival following Brefeldin A (BFA) induced stress. Treatment of wild type cells with either the TORC1 specific inhibitor rapamycin or the general TOR inhibitor Torin1 confirmed that a reduction in TORC1 activity promoted recovery from BFA induced stress. Phosphorylation of Ppk32 on two residues that are conserved within the SCYL pseudo-kinase family are required for this TOR inhibition. Phosphorylation on these sites controls Ppk32 protein levels and sensitivity to BFA. BFA induced ER stress does not account for the response to BFA that we report here, however BFA is also known to induce Golgi stress and impair traffic to lysosomes. In summary, Ppk32 reduce TOR signalling in response to BFA induced stress to support cell survival.

The Target of Rapamycin (TOR) pathway plays a central role coordinating cell growth and cell division in response to the different cellular environments. This is achieved by TOR controlling various metabolic processes, cell growth and cell division, and in part by the localisation of TOR protein complexes to specific internal endomembranes and compartments. Here, we report a novel role for the SCYL family pseudo-kinase, Ppk32 in restraining TOR signalling when cells experience stresses, which specifically affect endomembranes and compartments where TOR complexes are localised. Cells exposed to endomembrane stress (induced by Brefeldin A), displayed increased cell survival when simultaneously treated with the TOR complex 1 (TORC1) inhibitor, rapamycin, presumably because the reduction in TORC1 signalling slows cellular processes to allow cells sufficient time to recover and adapt to this stress. Importantly cancer, neuro-degeneration and neurological diseases are all associated with stress to the endomembrane protein trafficking system. Our findings suggest that therapeutic rapamycin treatment to reduce TOR signalling and block proliferation of cancer cells, which are inherently experiencing such stress, may have the unintended consequence of enhancing cell survival. It is notable, therefore, that our reported mechanisms to reduce Ppk32 protein levels, likely to be conserved in humans, may represent a way to increase TOR signalling and thus increase cell death of cancer types with inherent stress to these internal membrane systems.

Putative frameshift suppressors in Schizosaccharomyces pombe

Nine genetically distinct suppressors of ICR-170-induced ade6 and ade7 mutations have been identified in Schizosaccharomyces pombe. The nine suppressors of ICR-170-induced and spontaneous origin have been assigned to the three chromosomes by haploidization and meiotic analysis. They do not suppress missense or nonsense mutations and are therefore likely to be frameshift suppressors. Based on the spectrum of suppression, the nine suppressors fall into two mutually exclusive groups. Group I comprises the two dominant suppressors sufl and suf11. Group II consists of the seven dominant suppressors suf2 through suf8. The suppressors of both groups are inefficient and all lead to a marked reduction of growth rate. Within suppressor groups, combinations of suppressors lead to drastic reductions of growth rates and to an increased efficiency of suppression. Freely segregating modifiers of suppression increasing and decreasing the efficiency of supression have been found for all the suppressors. The two omnipotent suppressors sup1 and sup2 increase the efficiency of suppression of some frameshift suppressors. The suf5 locus is unstable and reverts at very high frequency both meiotically and mitotically.

Fission yeast Uve1 and Apn2 function in distinct oxidative damage repair pathways in vivo

In Schizosaccharomyces pombe, the endonuclease Uve1 functions as the first step in an alternate UV photo-product repair pathway that is distinct from nucleotide excision repair (NER). Based upon the broad substrate specificity of Uve1 in vitro, and the observation that Uve1 mutants accumulate spontaneous mutations at an elevated rate in vivo, we and others have hypothesized that this protein might have a function in a mutation avoidance pathway other than UV photo-product repair. We show here that fission yeast Uve1 also functions in oxidative damage repair in vivo. We have determined the spectrum of spontaneous mutations that arise in uve1 null (uve1 degrees ) cells and have observed that both G-->T(C-->A) and T-->G(A-->C) transversions occur at an increased rate relative to wildtype cells. These mutations are indicative of unrepaired oxidative DNA damage and are very similar to the mutation spectrum observed in 8-oxoguanine glycosylase (OGG1) mutants in Saccharomyces cerevisiae. We have generated an apn2 null (apn2 degrees ) strain and shown that it is mildly sensitive to H(2)O(2). Furthermore we have also shown that apn2 degrees cells have an elevated rate of spontaneous mutation that is similar to uve1 degrees. The phenotype of apn2 degrees uve1 degrees double mutants indicates that these genes define distinct spontaneous mutation avoidance pathways. While uve1 degrees cells show only a modest sensitivity to the oxidizing agent hydrogen peroxide (H(2)O(2)), both uve1 degrees and apn2 degrees cells also display a marked increased in mutation rate following exposure to H(2)O(2) doses. Collectively these data demonstrate that Uve1 is a component of multiple alternate repair pathways in fission yeast and suggest a possible role for Uve1 in a general alternate incision repair pathway in eukaryotes.

The effect of spermine on spontaneous and UV-induced mutations in Schizosaccharomyces pombe

The effect of different concentrations of spermine on spontaneous and UV-induced mutation in the adenine forward mutation system of Schizosaccharomyces pombe was investigated. The effect of spermine on spontaneous mutation was studied in 5 mutator strains (mut 1-4, mut 1-23, mut 2-9, mut 2-20 and mut 3-21) and on UV-induced mutation in a pigmented adenine-requiring strain and its radiation-sensitive derivative (rad 13). The effect of spermine exposure on mutation induction before and after UV irradiation was also investigated. Spermine increased spontaneous forward mutation in the mut 1-4 strain by 47%, and enhanced UV-induced forward mutation 2-fold in the rad 13 and normal pigmented strains. No antimutagenic effect of spermine was seen in any of the strains tested. This is in marked contrast to the antimutagenic effect of spermine observed with bacteria.

Chemical carcinogens transform BHK cells by inducing a recessive mutation

Treatment of BHK cells with mutagenic carcinogens induced neoplastic transformation in a single step. This transformation displayed the characteristics expected for a recessive mutation. Increasing doses of carcinogens induced transformants with kinetics similar to the kinetics with which they induced 6-thioguanine-resistant or ouabain-resistant mutants in the same population of cells. Transformants with temperature-restricted phenotypes were easily induced by carcinogens which cause mutations by base changes, but when ICR frameshift mutagens were used, the proportion of temperature-limited transformants was inversely related to the frequency with which a particular mutagen induced frameshift mutations. In hybrids between pseudodiploid isogenic strains of normal and transformed BHK cells, transformation was expressed as a dominant trait when the transformed parent was induced by a papovavirus, but was suppressed as a recessive trait when the transformed parent arose spontaneously or was chemically induced. Segregation of transformation was observed upon growth of suppressed normal hybrids, and the transformed phenotype which was reexpressed was in most cases characteristics of the original transformed parent.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Chemical carcinogens transform BHK cells by inducing a recessive mutation

Treatment of BHK cells with mutagenic carcinogens induced neoplastic transformation in a single step. This transformation displayed the characteristics expected for a recessive mutation. Increasing doses of carcinogens induced transformants with kinetics similar to the kinetics with which they induced 6-thioguanine-resistant or ouabain-resistant mutants in the same population of cells. Transformants with temperature-restricted phenotypes were easily induced by carcinogens which cause mutations by base changes, but when ICR frameshift mutagens were used, the proportion of temperature-limited transformants was inversely related to the frequency with which a particular mutagen induced frameshift mutations. In hybrids between pseudodiploid isogenic strains of normal and transformed BHK cells, transformation was expressed as a dominant trait when the transformed parent was induced by a papovavirus, but was suppressed as a recessive trait when the transformed parent arose spontaneously or was chemically induced. Segregation of transformation was observed upon growth of suppressed normal hybrids, and the transformed phenotype which was reexpressed was in most cases characteristics of the original transformed parent.

Extreme chemical mutagen sensitivity of respiratory adaptation in yeast

The respiratory adaptation process (i.e. essentially mitochondrial biogenesis) in the cells of both wild-type Saccharomyces cerevisiae and strains sensitive to ultraviolet radiation (UV) undergoing transition from the anaerobic to the aerobic state (1-2 h aeration) could be arrested by a prior incubation for 15--30 min with several chemical mutagens and other DNA-acting chemicals at very low concentrations (10-7 to 10-6 M added to cells suspended at the density of 10(7) cells/ml). At the same concentrations, these chemicals also inhibited DNA and RNA biosynthesis in maturing mitochondria during respiratory adaptation. This provides suggestive evidence for the view that the inhibitory effect of the chemical mutagens on respiratory adaptation could be due to lesions introduced into the DNA of promitochondria in the anaerobic cells. The system of respiratory adaptation in S. cerevisiae cells could serve as a rapid test for ascertaining the potentiality of a chemical to affect cellular DNA and probably, in turn, its potentiality to be mutagenic.

Studies on the mechanism of electron trasport in the bc1-segment of the respiratory chain in yeast. III. Isolation and characterization of an antimycin resistant mutant ANT 8 in Schizosaccharomyces pombe

1. A mutant (ANT 8) of Schizosaccharomyces pombe which shows resistance to antimycin both in vivo and in vitro is characterized biochemically and genetically. 2. In crosses of ANT 8 with auxotrophic strains, resistance to antimycin segregates 2:2 indicating that resistance is conferred by a single nuclear gene. Diploids heterozygous for the resistance gene, however, show segregation of the resistance and sensitivity during mitosis. Possible reasons for this segregation are discussed. 3. Compared with the wild type, the NADH oxidase of ANT 8 requires 13 times as much antimycin for 95% inhibition. After addition of ubiquinone-3, electron transport which is less sensitive to antimycin is found only in the mutant. 4. The resistance of the mutant ANT 8 si due to the much weaker binding of antimycin to mitochondria. As in the wild type, two antimycin binding sites can be separated by binding studies. From the inhibition curves it is evident that binding of antimycin to oxidized mitochondrial particles does not correspond with its inhibitory effect on the partly reduced enzyme in kinetic studies. 5. The peak of the b-cytochrome absorbing at 560.2 nm at 77 degrees K in the wild type is shifted to 561 nm in the mutant. 6. A special preparation method for mutant mitochondrial particles is described, yielding highly active enzymes and CO-insensitive cytochromes. 7. The results are discussed with reference to the components in our model of the respiratory chain, which may be responsible for this type of resistance.

Mutation and nuclear stage in Schizosaccharomyces pombe. II. Reverse mutations induced by x-rays in the absence of recombination

Induction of mutations by X-rays in Schizosaccharomyces pombe cells in which sister-strand recombination appears to be excluded is offered as evidence against a requirement for recombination in radiation-induced mutagenesis.