Ribosomal proteins in the fungus Podospora anserina: evidence for an electrophoretically altered 60S protein in a cycloheximide resistant mutant

Altered ribosomal protein L29 in a cycloheximide-resistant strain of Saccharomyces cerevisiae

A spontaneous high-level cycloheximide-resistant mutant of the yeast Saccharomyces cerevisiae (strain cy32) is found to have an altered protein of the large subunit (60S) of cytoplasmic ribosomes, namely protein L29. The resistance character segregates together with this biochemical defect and is semidominant in heterozygous diploids. Judged from in vitro susceptibility to inhibition by cycloheximide there are at least 50% resistant ribosomes present in such diploid strains. From these results it is concluded that cycloheximide resistance of mutant cy32 is caused by mutation of a single gene and that it is the structural gene for L29 which is affected.Preliminary genetic mapping data are also reported. They indicate a location of cyhx-32 marker on chromosome 7 near met13.

Elongation factor 2 and fragile X mental retardation protein control the dynamic translation of Arc/Arg3.1 essential for mGluR-LTD

Group I metabotropic glutamate receptors (mGluR) induce long-term depression (LTD) that requires protein synthesis. Here, we demonstrate that Arc/Arg3.1 is translationally induced within 5 min of mGluR activation, and this response is essential for mGluR-dependent LTD. The increase in Arc/Arg3.1 translation requires eEF2K, a Ca(2+)/calmodulin-dependent kinase that binds mGluR and dissociates upon mGluR activation, whereupon it phosphorylates eEF2. Phospho-eEF2 acts to slow the elongation step of translation and inhibits general protein synthesis but simultaneously increases Arc/Arg3.1 translation. Genetic deletion of eEF2K results in a selective deficit of rapid mGluR-dependent Arc/Arg3.1 translation and mGluR-LTD. This rapid translational mechanism is disrupted in the fragile X disease mouse (Fmr1 KO) in which mGluR-LTD does not require de novo protein synthesis but does require Arc/Arg3.1. We propose a model in which eEF2K-eEF2 and FMRP coordinately control the dynamic translation of Arc/Arg3.1 mRNA in dendrites that is critical for synapse-specific LTD.

Cycloheximide efflux in antibiotic-adapted cells of the fungus Mucor racemosus

Mucor racemosus cells adapted to either cycloheximide or trichodermin were approximately 40-fold more resistant to cycloheximide than nonadapted cells. Ribosomes isolated from adapted and nonadapted cells were equally sensitive to cycloheximide in an in vitro poly(U) translation assay. There was no detectable modification of cycloheximide by adapted cells. Uptake of drug by nonadapted and adapted cells was characterized by a rapid initial accumulation during the first 2 min of incubation with [3H]cycloheximide, followed by a steady-state intracellular drug concentration well below that of the medium. The steady-state drug concentration was approximately 10-fold lower in adapted cells than in nonadapted cells. Treatment of cells with sodium azide or dinitrophenol abolished the difference between uptake of drug by nonadapted and adapted cells and resulted in intracellular drug levels equal to that of the medium. Direct efflux measurements showed that adapted cells loaded with cycloheximide were able to excrete the drug far more rapidly than nonadapted cells. These results suggest that both nonadapted and adapted cells possess an energy-dependent efflux mechanism for transporting cycloheximide and that resistance in adapted cells is due to increased efficiency of transport.

At least seven ribosomal proteins are involved in the control of translational accuracy in a eukaryotic organism

In the filamentous fungus Podospora anserina, ribosomal proteins of 60 mutants impaired in the control of translational fidelity have been submitted to electrophoretic analysis. The "four corners" system combining four different two-dimensional polyacrylamide gel electrophoretic systems has been used. An altered electrophoretic pattern has been observed for 12 mutants. In mutants su3, su12 and su11 (decreased translational fidelity), proteins S1, S7 and S8, respectively, are altered. For AS mutants (increased translational fidelity), proteins S9, S12 and S19, respectively, are altered in AS9, AS1 and AS6 mutants, and protein S29 is lacking in AS3 mutants. The data suggest that five of these genes (at least) are the structural genes for the relevant proteins (su3:S1, su12:S7, AS1:S12, AS6:S19, AS9:S9), while the AS3 gene may code for a modifying enzyme.

Identification of the structural gene for the S9 ribosomal protein in the fungus Podospora anserina: a new protein involved in the control of translational accuracy

AS9-1 was isolated as a mutation restoring growth in a strain carrying the ribosomal mutation su12-1. The AS9-1 mutation confers a weak antisuppressor effect and a low level of resistance to paromomycin. Two-dimensional polyacrylamide gel electrophoresis patterns of the ribosomal proteins from AS9-1 strains show an altered S9 protein which is more basic than the wild-type form. The presence of the two forms of the protein (wild-type and mutant) in heterocaryotic strains strongly suggests that AS9 is the structural gene for the ribosomal protein S9.

Cloning and characterization of the rDNA repeat unit of Podospora anserina

DNA coding for ribosomal RNA in Podospora anserina has been cloned and was found as a tandemly repeated 8.3 kb sequence. The cloned rDNA was characterized by restriction endonuclease mapping. The location of 5.8S, 18S and 28S rRNA coding regions was established by DNA-RNA hybridization and S1 nuclease mapping. The organization of P. anserina rRNA genes is similar to that of Neurospora crassa and Aspergillus nidulans. The rDNA unit does not contain the sequence coding for 5S RNA.

Immunological comparison of individual ribosomal proteins in six species of the genus Podospora

The ribosomal proteins of six species belonging to the genus Podospora have been compared by electrophoretical and immunological methods. The important variability in the electrophoretical properties is similar to other lower eukaryotes but we could specify the extent of the variability of the r-proteins by the analysis of immunological relationships between individual proteins. Radioimmuno-detection of ribosomal proteins blotted on nitrocellulose sheets after separation on two-dimensional gels, was used to determine common antigenic sites on the r-proteins of the different species. These sites may be present on proteins that do or do not have identical electrophoretical mobilities. Furthermore, the proteins which co-migrate do not necessarily cross-react.

Isolation and characterization of cycloheximide-resistant mosquito cell clones

Following treatment of cultured mosquito cells (Aedes albopictus line of Singh) with the chemical mutagen ethyl methanesulfonate, we were able to isolate three cycloheximide-resistant clones. On the basis of growth kinetics, plating efficiency, and protein synthesis, these clones are 10- to 30-fold more resistant to cycloheximide than the parent cells. Cell-free lysates made from these cells retained 30-65% of their endogenous protein synthesizing ability in the presence of cycloheximide concentrations as high as 300 micrograms/ml. Protein synthesis in lysates from the parental cells, however, is reduced to about 10% of the control value (i.e., in the absence of drug) at 14 micrograms of cycloheximide/ml and was completely abolished at 75 micrograms/ml. These results indicate that cycloheximide resistance in these cells is likely due to an alteration in the protein synthetic machinery. This is the first description of cycloheximide-resistant insect cells, and the best example of cycloheximide resistance in cells originating from a higher eukaryotic organism.

Isolation and characterization of puromycin-resistant clones from cultured mosquito cells

We have isolated from an established Aedes albopictus (mosquito) cell line clones which are resistant to the antibiotic puromycin. On the basis of growth and plating efficiency, clones Pur-8026 and Pur-8612 were five- and seven-fold more resistant, respectively, to puromycin than wild-type cells. In vitro protein synthesis was resistant to puromycin only in extracts prepared from Pur-8612 cells. Measurements of puromycin transport, cross-resistance to colchicine, and sensitivity to Tween-80 indicating that resistance in Pur-8026 cells was due to membrane alteration(s) affecting permeability to puromycin. This is the first description of puromycin resistant in insect cells and also the first report of puromycin resistance in an animal cell variant associated with an alteration at the level of protein synthesis.

Mutations affecting translational fidelity in the eucaryote Podospora anserina: characterization of two ribosomal restrictive mutations

Fifty-nine mutations that restrict suppressor efficiency were selected in the fungus Podospora anserina using four different screening methods. Previous genetic analysis has shown that these antisuppressors lie in six loci and that they could be similar to ribosomal restrictive mutations known in Escherichia coli. The present study deals with the response of two of them, AS1-1 and AS6-1, to paromomycin and low temperature both in vivo and in vitro. The data demonstrate that ribosomes of the mutant and double-mutant strains are equally resistant to the ambiguity effect of paromomycin. These data are the first demonstration of mutations that increase translational fidelity in eucaryotic organism.

An actidione resistant Candida tropicalis from custard apple juice

An actidione resistant yeast, Candida tropicalis, was isolated from fermenting custard apple juice. Though a slight inhibition of growth was observed on the first day with 5000 ppm of actidione, growth was equal to control after one week. Sorbic acid at 500 ppm and above inhibited the growth of this yeast while sodium benzoate and potassium metabisulphite were unable to suppress the growth even at 1000 ppm. Fermentation and assimilation of different carbon sources were delayed in the presence of 1000 ppm of actidione suggesting the disruption of protein synthesis by actidione.

Genetics of ribosomal components in Podospora spp. as revealed by interspecific hybridization

The ribosomal proteins of several species within the genus Podospora have been analyzed by two-dimensional polyacrylamide gel electrophoresis. Differences in the migration of four proteins of the large subunit have been found between the two hybridizable species Podospora anserina and P. comata. Haploid offspring of this interspecific cross show all possible combinations of ribosomal proteins from both species. Therefore, the homologous proteins are functionally interchangeable. Moreover, the sizes of the different classes of offspring show that the genes encoding the four ribosomal proteins L2, L13, L16, and L20 are not clustered.

Search for ribosomal mutants in Podospora anserina: genetic analysis of cold-sensitive mutants

Twenty-four cold-sensitive (prototrophic) mutants were isolated after UV mutagenesis of protoplasts of the fungusPodospora anserina. Genetic analysis of these mutants was performed in order to detect those among them which were most likely to be impaired in translational fidelity. The 24 mutations belonged to 24 different genes. One half of the mutants were pleiotropic and displayed an altered phenotype: growth rate at the permissive temperature, germination of the spores, fertility and/or sporulation. Nine mutants differed from wild-type in their resistance levels to cycloheximide, trichodermin and/or paromomycin. Several mutations were linked to known ribosomal loci. Two mutations behaved like informational antisuppressors: one is allelic to the previously describedAs3gene and the other one defines a new antisuppressor gene,AS6.

Binding of cycloheximide to ribosomes from wild-type and mutant strains of Saccharomyces cerevisiae

Cycloheximide bound to cytoplasmic (80S) ribosomes of the yeast Saccharomyces cerevisiae with an association constant (Ka) of 2.0 (+/- 0.5) x 10(7) M-1. The number of binding sites found per ribosome was between 0.4 and 0.6; it was reduced by high-salt treatment of ribosomes 60S particles prepared in the presence of high salt had a lower affinity (Ka: 5.5 [+/- 0.5] x 10(6) M-1) than did 80S ribosomes, but a greater proportion of particles (0.8) were able to bind. No specific binding to 40S subunits was observed. The addition of supernatant fractions (S100, high-salt wash fraction) increased the number of binding sites found per 80S ribosome up to 0.8, leaving the association constant unchanged. In contrast, the affinity of 60S subunits was enhanced to a Ka value of 3.5 x 10(-7) M-1 by the addition of supernatant fractions, whereas the number of binding sites stayed constant. A model to explain these facts is proposed. 80S ribosomes, as well as 60S subunits of strain cy32, which is highly resistant to cycloheximide and altered in ribosomal protein L29 (18), showed a drastically reduced affinity for the drug (Ka values of 2.0 x 10(6) M-1).

A new mutant form of the ribosomal protein L21 in the fungus Podospora anserina: identification of the structural gene for this protein

A muation in the cyR1 gene of the fungus Podospora anserina confers resistance to cycloheximide and leads to an alteration of the 60S ribosomal protein L21 (Bégueret et al. 1977). Nine revertants of this mutant were isolated and the properties of these strains were analyzed. It was found that one revertant strain contains a new mutant form of L21. It is proposed that cyR1 gene is the structural gene for protein L21 and that the alteration of this protein is responsible for the resistance to cycloheximide in vivo.

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.

Resistance against cycloheximide in cell lines from Chinese hamster and human cells is conferred by the large subunit of cytoplasmic ribosomes

Cell lines from Chinese hamster ovary [CHO-K1-D3] and human fibroblast cells [46, XX, 18p-] were mutagenized with N-nitrosomethylurea followed by a selection for cycloheximide resistance. Two mutants resistant against the drug were selected from either wildtype. 80S ribosomes and their ribosomal subunits were isolated from all mutant and wildtype cells. 80S ribosomes reassociated from the isolated subunits were as active as isolated 80S couples in the poly (U) dependent poly (Phe) synthesis. Hybrid 80S ribosomes constructed from subunits of the various cell lines of the same species were fully active, whereas the interspecies 80S hybrids were not active at all in poly (Phe) synthesis. Hybrid 80S ribosomes from subunits of mutant and the corresponding wildtype cells were tested in the poly (U) assay in the presence and absence of cycloheximide. The results strikingly indicate that in all four mutant cell lines the resistance against cycloheximide is conferred by the large subunit of cytoplasmic ribosomes.

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.

Genetic and biochemical analysis of cycloheximide resistance in the fungus Podospora anserina

Genetic analysis of cycloheximide-resistant mutants has shown that at least three genes control the resistance to cycloheximide in Podospora anserina and that the antibiotic resistance is recessive to sensitivity. In vitro and in vivo studies of protein synthesis indicated that for two mutants cycloheximide resistance is associated with the ribosomes. For one of these mutants, the elongation step in protein biosynthesis is insensitive to cycloheximide over a wide range of concentration. In this mutant the resistance to cycloheximide is a property of the 60S subunit.