Genetic studies on cycloheximide-resistant strains of Schizosaccharomyces pombe

Loss of Msp1p in Schizosaccharomyces pombe induces a ROS-dependent nuclear mutator phenotype that affects mitochondrial fission genes

Mitochondria continually fuse and divide to dynamically adapt to changes in metabolism and stress. Mitochondrial dynamics are also required for mitochondrial DNA (mtDNA) integrity; however, the underlying reason is not known. In this study, we examined the link between mitochondrial fusion and mtDNA maintenance in Schizosaccharomyces pombe, which cannot survive without mtDNA, by screening for suppressors of the lethality induced by loss of the dynamin-related large GTPase Msp1p. Our findings reveal that inactivation of Msp1p induces a ROS-dependent nuclear mutator phenotype that affects mitochondrial fission genes involved in suppressing mitochondrial fragmentation and mtDNA depletion. This indicates that mitochondrial fusion is crucial for maintaining the integrity of both mitochondrial and nuclear genetic information. Furthermore, our study suggests that the primary roles of Msp1p are to organize mitochondrial membranes, thus making them competent for fusion, and maintain the integrity of mtDNA.

High-throughput genetic interaction mapping in the fission yeast Schizosaccharomyces pombe

Epistasis analysis, which reports on the extent to which the function of one gene depends on the presence of a second, is a powerful tool for studying the functional organization of the cell. Systematic genome-wide studies of epistasis, however, have been limited, with the majority of data being collected in the budding yeast, Saccharomyces cerevisiae. Here we present two 'pombe epistasis mapper' strategies, PEM-1 and PEM-2, which allow for high-throughput double mutant generation in the fission yeast, S. pombe. These approaches take advantage of a previously undescribed, recessive, cycloheximide-resistance mutation. Both systems can be used for genome-wide screens or for the generation of high-density, quantitative epistatic miniarray profiles (E-MAPs). Since S. cerevisiae and S. pombe are evolutionary distant, this methodology will provide insight into conserved biological pathways that are present in S. pombe, but not S. cerevisiae, and will enable a comprehensive analysis of the conservation of genetic interaction networks.

Selective inhibition of MAPKK Wis1 in the stress-activated MAPK cascade of Schizosaccharomyces pombe by novel berberine derivatives

Intracellular molecular targets of novel berberine derivatives, HWY 289 and HWY 336, were identified by a screen of a variety of mutants in fission yeast Schizosaccharomyces pombe. HWY 289 and HWY 336 completely inhibited the proliferation of wild type as well as various mutant fission yeast cells (minimal inhibitory concentrations were 29.52 microm for HWY 289 and 11.83 microm for HWY 336), but did not affect the proliferation of Wis1 mitogen-activated protein kinase kinase (MAPKK) deletion mutants. In addition, HWY 289 with an IC(50) value of 7.3 microm or HWY 336 with IC(50) of 5.7 microm specifically inhibited in vitro kinase activities of purified Wis1, whereas either compound did not affect the activities of other kinases in the mitogen-activated protein kinase (MAPK) cascades of fission yeast. These genetic and biochemical results demonstrate the high degree of specificity of HWY 289 and HWY 336 to MAPKK Wis1 and suggest that the cytotoxicity of these compounds is not simply due to the inhibition of Wis1 kinase activity. High salt wash experiments have shown that strong noncovalent binding occurs between Wis1 and either HWY 289 or HWY 336. The preincubation of Wis1 kinase with ATP did not affect the inhibition of Wis1 by HWY 289 and HWY 336, but when Wis1 was preincubated with MBP, a protein substrate, Wis1 kinase activity was no longer inhibited. These observations demonstrate that HWY 289/HWY 336 do inhibit Wis1 kinase, not by binding to the ATP-binding site but by disturbing the binding of substrate to the kinase. Target validation of the complex of HWY 289/HWY 336 and Wis1 kinase will provide important clues for the mechanism of specific cytotoxicity of these compounds in S. pombe. On a broader aspect, it would create an initiative to further modify and develop compounds that selectively inhibit kinases and cause cytotoxicity in various MAPK cascades including those of mammals.

The amiloride resistance gene, car1, of Schizosaccharomyces pombe

Amiloride, an inhibitor of various sodium transporters, is toxic to Schizosaccharomyces pombe at low concentration in minimal but not in rich media. Amiloride-resistant mutants were isolated and shown to represent a new locus (car1 for changed amiloride resistance) on chromosome I. The car1 gene was cloned and sequenced. Sequence analysis revealed an open reading frame of 526 amino acids with a predicted molecular weight of 58,545 Da. It has 52% hydrophobic residues and belongs to the class of 12-transmembrane-domain transport proteins. Gene disruption of car1 results in increased amiloride resistance. car1 has sequence similarity to proteins from Candida associated with resistance to benomyl, methotrexate and cycloheximide. No single physiologically identifiable component of sodium transport appeared to be lost. We propose that car1 serves an uptake function, perhaps as a symport with an unknown substrate and this carrier may transport amiloride into the cell. Further, we suggest that amiloride toxicity at low concentrations is not due to its effect on sodium transport but, rather, depends on intracellular interference with an unknown biosynthetic pathway.

Drug resistance in human pathogenic fungi

Since the therapy of the mycoses, particularly the systemic mycoses, is relatively long-term in nature, emergence of resistance to antifungal drugs during the treatment of period would be of considerable clinical importance. However, most reports of resistance to antifungal agents among human pathogenic fungi indicate that naturally-occurring resistance is very rare, and that the induction of resistant mutants or variants is much more difficult to achieve in vitro and in vivo than with bacteria. As a matter of fact, amphotericin B and some other classic antifungals have not as yet posed a broadly significant problem relative to drug resistance despite their widespread and frequent use. Fungal resistance has thus received little attention, in contrast to the critical importance of bacterial resistance frequently caused by a variety of antibacterial chemotherapeutic agents, until a single exception to this generalization arose with the advent of flucytosine. This new development has aroused great interest in the problem of fungal resistance among the scientists involved with medical mycology. It is generally believed that fungi, like bacteria, are intrinsically capable of developing resistance to antifungal agents. As illustrated by flucytosine, inherently resistant mutants to antifungals occur within sensitive strains of human pathogenic fungi with significant frequency. Given the relatively high degree of such primary resistance, these mutants should develop secondary resistance during therapy, thus resulting in considerable limitations in the clinical usefulness of the antifungals. Virtually, all unsuccessful cases of mycoses treated with some of the recently exploited antifungal drugs, albeit scarce to date, would obviously be attributable to the occurrence of secondary resistance. The exploitation of new antifungal drugs thus requires investigations of their resistance as one of the most important research projects to be undertaken before receiving approval for use on humans. This paper reviews from various aspects the literature on resistance to various classic and novel antifungal agents among human pathogenic fungi. The resistance of some nonpathogenic fungi to these agents will also be described from genetic and biochemical points of view.

Gene database for the fission yeast Schizosaccharomyces pombe

As an aid to the fission yeast genome project, we describe a database for Schizosaccharomyces pombe consisting of both genetic and physical information. As presented, it is therefore both an updated gene list of all the nuclear genes of the fission yeast, and provides an estimate of the physical distance between two mapped genes. Additionally, a field indicates whether the sequence of the gene is available. Currently, sequence information is available for 135 of the 501 known genes.

The cytoplasmic ribosomes of Chlamydomonas reinhardtii: characterization of antibiotic sensitivity and cycloheximide-resistant mutants

In vitro protein synthesis was used to characterize the antibiotic sensitivity of cytoplasmic ribosomes from wild-type and antibiotic-resistant strains of Chlamydomonas reinhardtii. Cytoplasmic ribosomes from two cycloheximide-resistant mutants, act-1 and act-2, were resistant to the antibiotic in vitro. The alteration effected by the act-1 mutation, which was dominant in diploids, was localized to the large subunit of the cytoplasmic ribosomes, but no ribosomal protein alterations were detected using two-dimensional gel electrophoresis. The act-2 mutation, which was semidominant in diploids, was frequently associated with a charge alteration in the large subunit ribosomal protein (r-protein) cyL38 that segregated independently from the antibiotic-resistant phenotype in crosses.

Pleiotropic changes in cycloheximide-resistant insect cell clones

Somatic cell mutants resistant to drugs that interact with the eukaryotic ribosome provide a useful tool for studies on ribosome structure, function, and genetics. From Aedes albopictus (mosquito) cells, cycloheximide-resistant mutants (Cx-705 and Cx-738) that were about 30-fold more resistant to cycloheximide than the parental cells have been obtained. The observation that protein synthesis in cell-free lysates from Cx-705 and Cx-738 cells was resistant to cycloheximide led us to suspect that the alteration in these mutants might affect the ribosome. The present studies show that the cycloheximide-resistant cells grow poorly and eventually die at 34.5 degrees C, a temperature at which wild-type cells grow normally. Relative to control cells, the cycloheximide-resistant cells show increased sensitivity to G-418, another antibiotic that interacts with the eukaryotic ribosome. However, there were no differences between cycloheximide-resistant cells and wild-type cells in sensitivity to puromycin, emetine, or cryptopleurine. Cx-705 cells were predominantly diploid; in contrast, the frequency of tetraploid nuclei in Cx-738 cells was about 40%.

Multiple drug resistance in the fission yeast Schizosaccharomyces pombe: evidence for the existence of pleiotropic mutations affecting dependent transport systems

The uptake of L-tyrosine into wild type and antibiotic resistant strains of Schizosaccharomyces pombe requires an energy source, is initially linear with respect to time, is inhibited by 2,4-dinitrophenol and sodium azide and is saturable. However the initial uptake rates and the amount of L-tyrosine accummulated by antibiotic resistant strains are much less than wild type. Comparison of the kinetic constants of uptake shows that mutant strains have a reduced maximum velocity of uptake compared to wild type and a larger Km. Since the three mutant strains possess a permeability barrier to L-tyrosine as well as being drug resistant this is an indication that antibiotic resistance may be caused by a decrease in plasma membrane permeability.

Nucleo-cytoplasmic interactions in the petite negative yeast Schizosaccharomyces pombe. Inhibition of nuclear and mitochondrial DNA syntheses in the absence of cytoplasmic protein synthesis

In the petite positive yeast, Saccharomyces cerevisiae, cycloheximide selectively inhibits protein synthesis on cytoplasmic ribosomes, and, as a consequence, nuclear DNA synthesis. Mitochondrial DNA, however, is synthesized for 4-6 h after cessation of protein synthesis. In this paper we show that in contrast to Saccharomyces cerevisiae, synthesis of mitochondrial and nuclear DNA is tightly coordinated in the petite negative yeast Schizosaccharomyces pombe, since inhibition of cytoplasmic protein synthesis leads immediately to cessation of both nuclear and mitochondrial DNA synthesis.

Cycloheximide resistance of Physarum polycephalum

In the presence of cycloheximide, wild-type plasmodia of Physarum polycephalum exhibit an immediate decrease in deoxyribonucleic acid synthesis, a reduction in the incorporation of [3H]thymidine into thymidine triphosphate, and an increase in the level of thymidine triphosphate, as well as a decrease in protein synthesis. In this study, we have utilized a cycloheximide-resistant (Cycr) amoebic strain selected from a population of cells mutagenized with nitrosoguanidine. Segregation data indicate that the resistance is due to a single mutation. We have used this Cycr mutant to construct Cycr plasmodial strains. Ribosomes isolated from such Cycr plasmodia showed resistance to cycloheximide in vitro, in contrast to ribosomes isolated from wild-type plasmodia. The Cycr plasmodia showed none of the cycloheximide-induced biochemical effects. Plasmodia heterozygous for the resistance marker were sensitive to cycloheximide with regard to growth but showed an intermediate response in the biochemical parameters. Heterokaryons formed by fusion of various proportions of the sensitive and resistant plasmodia showed a resistance with regard to both growth and biochemical parameters which was directly related to the fraction of Cycr plasmodia present in the heterokaryons. The data are consistent with the hypothesis that the effects of cycloheximide on deoxyribonucleic acid synthesis and nucleoside metabolism are secondary to the effect of the drug on protein synthesis in this organism.

Characterisation of ribosomes from drug resistant strains of Schizosaccharomyces pombe in a poly U directed cell free protein synthesising system

Mutants of Schizosaccharomyces pombe were isolated as resistant either to trichodermin or to anisomycin. Growth tests showed that the majority of mutants isolated were cross resistant to both drugs and also to cycloheximide. A limited genetic analysis showed that mutants at least four loci, tri3, tri4, ani1 and ani2, had this phenotype as was also the case for mutants at three cycloheximide resistant loci, cyh2, cyh3 and cyh4 reported previously (Ibrahim and Coddington, 1976). Allelism tests showed that the tri3, ani2 and cyh4 strains were allelic. A mutant at another trichodermin resistant locus, tri5, was cross resistant to anisomycin but sensitive to cycloheximide. Ribosomes from wild type and selected strains were analysed in a poly U directed cell free protein synthesising system. Three strains, cyh1-C7, ani1-F1 and tri-N15 (probably a tri5 allele) possessed ribosomes which were more resistant than the wild type to the drugs used in their isolation. In each case the site of the resistance was in the 60S subunit. Ribosomes from the cyh2, cyh3 and cyh4 strains were as sensitive to cycloheximide as those from wild type.

Cold-sensitivity of a double mutant strain combining two ribosomal mutations in the ascomycete Podospora anserina

A double mutant strain combining two ribosomal mutations conferring resistance to cycloheximide exhibits a cold-sensitive phenotype. At low temperature the biosynthesis of the 60S subunit is impaired. Genetic analysis of cold-resistant revertants have shown that this double mutant strain can be used efficiently to isolate new ribosomal mutations.

Genetical studies on revertants to sensitivity from a cycloheximide resistant strain of Schizosaccharomyces pombe

Six UV induced cycloheximide-sensitive revertants were isolated from the cyh1-C7 strain of Schizosaccharomyces pombe which is resistant to cycloheximide. In all cases reversion to sensitivity was due to a forward mutation in a second suppressor gene. Genetical analysis showed that at least two genes, designated scr1 and scr2 (scr=suppression of cycloheximide resistance) were involved. Both scr1 and scr2 suppressed the resistance of six independently isolated alleles at the cyh1 locus. They had no effect on two known nonsense mutations in the ade7 locus. The cyh1-C7 strain has an altered 60S ribosomal protein which can be detected by two-dimensional polyacrylamide gel electrophoresis. In two suppressed strains, cyh1-C7 scr1 and cyh1-C7 scr2, the original altered protein was present. However no further ribosomal protein differences were observed which could be correlated with the presence of the scr genes. Both scr mutations conferred cold sensitivity on the organism indicating that they were of the missense type. Hence it seems certain that scr1 and scr2 are not mutations in tRNA genes leading to either nonsense or missense suppression. There is however no direct evidence that they code for ribosomal proteins and exert their effect on cyh1-C7 at the ribosomal level.

Ribosomal suppressors and antisuppressors in Podospora anserina: resistance to cycloheximide

Informational suppressors and antisuppressors have been previously isolated in Podospora anserina, and a range of exclusively genetic arguments have led to the assumption that they correspond to ribosomal mutations. An in vivo and in vitro comparison of the effect of the ribosomal inhibitor cycloheximide on wildtype and mutant strains described in this paper confirms the ribosomal hypothesis for at least some mutants. Indeed, the four mutants in the AS3 gene were cycloheximide resistant, and their ribosomes were found to be resistant when analyzed by polyuridyl-directed polyphenylalanine systhesis. On the other hand, ribosomes from two su 1 mutants were hypersensitive to the drug.

Identifying sites of simultaneous DNA replication in eukaryotes by N-methyl-N'-nitro-N-nitrosoguanidine multiple mutagenesis

N-methyl-N'-nitro-N-nitrosoguanidine (NG) induces certain classes of multiple mutations in yeast at high frequency. By selecting for mutation at one locus (his4 or leu1) one frequently obtains double mutants where another mutation to temperature sensitivity has also been induced. This multiple mutagenesis exhibits a considerable specificity: for mutation at one particular locus there is a high chance that another mutation will be found in the same cell at one of a restricted number of other loci. For any given locus (e.g. his4) there is a spectrum of sites at which temperature-sensitivity mutations are co-induced. This spectrum differs for different loci, such that the spectrum of sites co-mutating with leu1 differs completely from that for sites co-mutating with his4. This NG'induced co-mutation is interpreted in terms of NG acting to enhance mutagenesis at sites of simultaneous DNA replication within the cell. The results so obtained indicate a very strict control over the order and timing of gene replication in Saccharomyces cerevisiae, and it is suggested that it is now possible to use NG double mutagenesis to try and locate origins of replication in yeast.