芽殖酵母和裂殖酵母中异染色质形成机制

芽殖酵母(Saccharomyces cerevisiae)和裂殖酵母(Schizosaccharomyces pombe)是用来研究异染色质形成、细胞周期、DNA复制等重要细胞功能的理想单细胞真核生物.本文主要介绍这2种酵母中异染色质形成的机制.异染色质是一种抑制基因转录和DNA重组的特殊染色质结构.尽管在芽殖酵母和裂殖酵母中异染色质形成都需要组蛋白修饰,但异染色质建立的机制不同.在芽殖酵母中参与异染色质形成的主要蛋白是Sir1-4蛋白（其中Sir2为组蛋白H3去乙酰化酶）,而组蛋白H3赖氨酸9甲基化酶Clr4和异染色质蛋白Swi6在裂殖酵母异染色质形成中起关键的作用.在这两个酵母中,参与异染色质形成的组蛋白修饰蛋白由DNA结合蛋白招募到异染色质.此外,裂殖酵母也利用RNA干扰系统招募组蛋白修饰蛋白.     

Versatile use of Schizosaccharomyces pombe plasmids in Saccharomyces cerevisiae

The two model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe appear to have diverged 1000 million years ago. Here, we describe that S.?pombe vectors can be propagated efficiently in S.?cerevisiae as pUR19 derivatives, and the pREP and pJR vector series carrying the S.?cerevisiae LEU2 or the S.?pombe ura4(+) selection marker are maintained in S.?cerevisiae cells. In addition, genes transcribed from the S.?pombe nmt1(+) promoter and derivatives are expressed in budding yeast. Thus, S.?pombe vectors can be used as shuttle vectors in S.?cerevisiae and S.?pombe. Our finding greatly facilitates the testing for functional orthologs of protein families and simplifies the cloning of new S.?pombe plasmids by using the highly efficient in vivo homologous recombination activity of S.?cerevisiae.     

Expression of human antithrombin III in Saccharomyces cerevisiae and Schizosaccharomyces pombe

Recombinant plasmids were constructed that direct the synthesis of human antithrombin III in baker's yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe. The signal sequence of antithrombin III was recognized by both yeast species, and antithrombin III was secreted into the medium. When the signal sequence was replaced by a sequence of ten arbitrary amino acids, the product expressed from such a construct stayed inside the cell. Antithrombin III was glycosylated by the baker's and fission yeast and was immunologically identical to antithrombin III isolated from human plasma. Antithrombin III isolated from the culture media of recombinant yeasts was biologically active, as could be shown by progressive inhibitor activity and heparin cofactor activity.     

Characterization of Schizosaccharomyces pombe malate permease by expression in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, L-malic acid transport is not carrier mediated and is limited to slow, simple diffusion of the undissociated acid. Expression in S. cerevisiae of the MAE1 gene, encoding Schizosaccharomyces pombe malate permease, markedly increased L-malic acid uptake in this yeast. In this strain, at pH 3.5 (encountered in industrial processes), L-malic acid uptake involves Mae1p-mediated transport of the monoanionic form of the acid (apparent kinetic parameters: Vmax = 8.7 nmol/mg/min; Km = 1.6 mM) and some simple diffusion of the undissociated L-malic acid (Kd = 0.057 min(-1)). As total L-malic acid transport involved only low levels of diffusion, the Mae1p permease was further characterized in the recombinant strain. L-Malic acid transport was reversible and accumulative and depended on both the transmembrane gradient of the monoanionic acid form and the DeltapH component of the proton motive force. Dicarboxylic acids with stearic occupation closely related to L-malic acid, such as maleic, oxaloacetic, malonic, succinic and fumaric acids, inhibited L-malic acid uptake, suggesting that these compounds use the same carrier. We found that increasing external pH directly inhibited malate uptake, resulting in a lower initial rate of uptake and a lower level of substrate accumulation. In S. pombe, proton movements, as shown by internal acidification, accompanied malate uptake, consistent with the proton/dicarboxylate mechanism previously proposed. Surprisingly, no proton fluxes were observed during Mae1p-mediated L-malic acid import in S. cerevisiae, and intracellular pH remained constant. This suggests that, in S. cerevisiae, either there is a proton counterflow or the Mae1p permease functions differently from a proton/dicarboxylate symport.     

Cloning and expression of a Saccharomyces diastaticus glucoamylase gene in Saccharomyces cerevisiae and Schizosaccharomyces pombe.

A recombinant plasmid pool of the Saccharomyces diastaticus genome was constructed in plasmid YEp13 and used to transform a strain of Saccharomyces cerevisiae. Six transformants were obtained which expressed amylolytic activity. The plasmids each contained a 3.9-kilobase (kb) BamHI fragment, and all of these fragments were cloned in the same orientations and had identical restriction maps, which differed from the map of the STA1 gene (I. Yamashita and S. Fukui, Agric. Biol. Chem. 47:2689-2692, 1983). The glucoamylase activity exhibited by all S. cerevisiae transformants was approximately 100 times less than that of the donor strain. An even lower level of activity was obtained when the recombinant plasmid was introduced into Schizosaccharomyces pombe. No expression was observed in Escherichia coli. The 3.9-kb BamHI fragment hybridized to two sequences (4.4 and 3.9 kb) in BamHI-digested S. diastaticus DNA, regardless of which DEX (STA) gene S. diastaticus contained, and one sequence (3.9 kb) in BamHI-digested S. cerevisiae DNA. Tetrad analysis of crosses involving untransformed S. cerevisiae and S. diastaticus indicated that the 4.4-kb homologous sequence cosegregated with the glucoamylase activity, whereas the 3.9-kb fragment was present in each of the meiotic products. Poly(A)+ RNA fractions from vegetative and sporulating diploid cultures of S. cerevisiae and S. diastaticus were probed with the 3.9-kb BamHI fragment. Two RNA species, measuring 2.1 and 1.5 kb, were found in both the vegetative and sporulating cultures of S. diastaticus, whereas one 1.5-kb species was present only in the RNA from sporulating cultures of S. cerevisiae.     

Inhibition of mitochondrial protein synthesis in vivo by erythromycin in Schizosaccharomyces pombe and Saccharomyces cerevisiae

Summary In the presence of erythromycin (0.01 mg/ml) growth of Schizosaccharomyces pombe in non-fermentable substrate (glycerol) is reduced to 5–15% of the control without erythromycin, whereas growth in fermentable substrate (5% glucose) is left unaffected by concentrations up to 5 mg/ml. The reduction of growth under derepressed conditions is paralleled by inhibition of the formation of cytochromes a·a₃ and b. Mitochondrial protein synthesis is inhibited to about 50% in Schizosaccharomyces pombe and to about 90% in Saccharomyces cerevisiae. These results support the hypothesis that inhibition of mitochondrial protein synthesis is the primary effect of erythromycin.     

Motifs in Schizosaccharomyces pombe ars3002 important for replication origin activity in Saccharomyces cerevisiae

Ars3002 is an efficient single-copy replication origin in the fission yeast, Schizosaccharomyces pombe. In a previous study, we tested the effects of consecutive approximately 50-bp deletions throughout ars3002 on the replication efficiency of those origins in S. pombe. Here we report the results of our use of the same approximately 50-bp deletions to test the hypothesis that some of the cis-acting sequences important for replication origin activity in fission yeast might be conserved in the evolutionarily distant budding yeast, Saccharomyces cerevisiae. We found that in most cases there was no correlation between the effects of particular mutations in S. pombe and in S. cerevisiae. We conclude that it is unlikely that any of the cis-acting sequences recognised by homologous replication proteins is conserved between these two yeast species.     

Bioinformatic analysis of the link between gene composition and expressivity in Saccharomyces cerevisiae and Schizosaccharomyces pombe

The compositional non-randomness was studied in genes of Saccharomyces cerevisiae and Schizosaccharomyces pombe. In both species, codon usage is well correlated with expressivity (measured as the codon adaptation index). Both species generally display higher nucleotide non-randomness in the group of highly expressed genes than in the lowly expressed genes. The highly expressed genes in both species are furthermore characterized by marked peaks in non-randomness at N=3 upstream of start codons, N=2 downstream of start codons and at N=1 and N=7 downstream of stop codons, indicating that these nucleotides may be key elements in translational regulation. Intragenic variation in codon usage was also observed to be linked to expressivity. It is suggested that the firm link between expressivity and codon usage calls for codon optimization. Based on bioinformatic calculations, examples of proteins are given for which codon optimizations might be relevant.     

A comparative analysis of an orthologous proteomic environment in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe

The sequential application of protein tagging, affinity purification, and mass spectrometry enables highly accurate charting of proteomic environments by the characterization of stable protein assemblies and the identification of subunits that are shared between two or more protein complexes, termed here "proteomic hyperlinks." We have charted the proteomic environments surrounding the histone methyltransferase, Set1, in both yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Although the composition of these nonessential Set1 complexes is remarkably conserved, they differ with respect to their hyperlinks to their proteomic environments. We speculate that conservation of the core components of protein assemblies and variability of hyperlinks represents a general principle in the molecular organization of eukaryotic proteomes.     

Functional Expression of Schizosaccharomyces pombe Vba2p in the Vacuolar Membrane of Saccharomyces cerevisiae

A vacuolar membrane protein, Vba2p of Schizosaccharomyces pombe, is involved in basic amino acid uptake by intact cells. Here we found evidence that Vba2p mediated ATP-dependent lysine uptake by vacuolar membrane vesicles of Saccharomyces cerevisiae. Vba2p was also responsible for quinidine sensitivity, and the addition of lysine improved cell growth on quinidine-containing media. These findings should be useful for further characterization of Vba2p.     

Transformation of Saccharomyces cerevisiae and Schizosaccharomyces pombe with linear plasmids containing 2 micron sequences.

Linear plasmids were constructed by adding telomeres prepared from Tetrahymena pyriformis rDNA to a circular hybrid Escherichia coli-yeast vector and transforming Saccharomyces cerevisiae. The parental vector contained the entire 2 mu yeast circle and the LEU gene from S. cerevisiae. Three transformed clones were shown to contain linear plasmids which were characterized by restriction analysis and shown to be rearranged versions of the desired linear plasmids. The plasmids obtained were imperfect palindromes: part of the parental vector was present in duplicated form, part as unique sequences and part was absent. The sequences that had been lost included a large portion of the 2 mu circle. The telomeres were approximately 450 bp longer than those of T. pyriformis. DNA prepared from transformed S. cerevisiae clones was used to transform Schizosaccharomyces pombe. The transformed S. pombe clones contained linear plasmids identical in structure to their linear parents in S. cerevisiae. No structural re-arrangements or integration into S. pombe was observed. Little or no telomere growth had occurred after transfer from S. cerevisiae to S. pombe. A model is proposed to explain the genesis of the plasmids.     

Expression of a VEGF-like protein from Parapoxvirus ovis in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe

We report on the expression of a VEGF-like protein encoded by Parapoxvirus ovis in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. We show that a lysine residue at amino acid position 2 (K2) is an important determinant for the stability of this protein in S. cerevisiae. Replacement of K2 by an arginine results in stabilization of the protein. This observation suggests that this lysine may be a target for ubiquitinylation, which is a prerequisite for proteasome-mediated protein degradation. Interestingly, in S. pombe the lysine (K2) has no influence on the stability of the protein. This result indicates that the two yeast species exhibit significant differences in their protein degradation pathways.     

Cloning of Schizosaccharomyces pombe bio2 by Heterologous Complementation of a Saccharomyces cerevisiae Mutant

A Saccharomyces cerevisiae mutant affected in the last step of the biotin biosynthesis pathway was isolated by using a transposon mutagenesis method. The gene BIO2, encoding a biotin synthase, is shown to be interrupted in this mutant. Heterologous complementation experiment allowed the cloning and the characterization of a novel bio gene: bio2, encoding biotin synthase from Schizosaccharomyces pombe. Received: 4 June 1999 / Accepted: 12 July 1999     

Characterization of a Saccharomyces cerevisiae homologue of Schizosaccharomyces pombe Chk1 involved in DNA-damage-induced M-phase arrest

Chk1 is an evolutionarily conserved protein kinase that plays an essential role in mediating G2 arrest in response to DNA damage in Schizosaccharomyces pombe and human cells. It functions by maintaining the inhibition (by phosphorylation of a specific tyrosine residue) of the cyclin-dependent kinase Cdc2 that initiates the G2/M transition. Here, we characterize a structural homologue of Chk1 in the budding yeast Saccharomyces cerevisiae. In this organism, G2/M arrest following DNA damage is considered to be independent of tyrosine phosphorylation of the Cdc2 homologue Cdc28. Nevertheless, a partial defect in G2/M-phase arrest following treatment with ionizing radiation, but not UV radiation, is associated with deletion of CHK1. The fact that such an effect remains detectable in cells synchronized with the microtubule inhibitor nocodazole prior to γ irradiation implies the existence of a CHK1-dependent checkpoint in M phase. We conclude from epistasis analysis that Chk1 participates in the Pds1-dependent subpathway of M-phase arrest. In spite of the partial checkpoint defect of the chk1 mutant, the survival of colony-forming cells is not notably decreased following UV and γ irradiation. In two-hybrid screens, we identified a heme-binding stress protein (encoded by the yeast ORF YNL234W), a protein involved in genomic silencing (Sas3) and Chk1 itself as interacting partners of Chk1. Received: 7 July 1999 / Accepted: 29 October 1999     

Atomic force microscopic study of the influence of physical stresses on Saccharomyces cerevisiae and Schizosaccharomyces pombe

Morphological changes in the cell surfaces of the budding yeast Saccharomyces cerevisiae (strain NCYC 1681), and the fission yeast Schizosaccharomyces pombe (strain DVPB 1354), in response to thermal and osmotic stresses, were investigated using an atomic force microscope. With this microscope imaging, together with measurements of culture viability and cell size, it was possible to relate topological changes of the cell surface at nanoscale with cellular stress physiology. As expected, when the yeasts were exposed to thermostress or osmostress, their viability together with the mean cell volume decreased in conjunction with the increase in thermal or osmotic shock. Nevertheless, the viability of cells stressed for up to 1 h remained relatively high. For example, viabilities were >50% and >90% for the thermostressed, and >60% and >70% for the osmostressed S. cerevisiae and Schiz. pombe, respectively. Mean cell volume measurements, and bearing and roughness analyses of atomic force microscope images of stressed yeasts indicate that Schiz. pombe may be more resistant to physical stresses than S. cerevisiae. Overall, this study has highlighted the usefulness of atomic force microscope in studies of yeast stress physiology.