Jürg Kohli

Genetic engineering of Schizosaccharomyces pombe: a system for gene disruption and replacement using the ura4 gene as a selectable marker.

SummaryA system is described for gene disruption and replacement in Schizosaccharomyces pombe based on the homologous selectable marker, ura4, the structural gene for orotidine-5′-phosphate decarboxylase. The presence of a single copy of the wild-type gene can rescue a ura4 auxotrophic mutant. Furthermore, ura4−cells can be selected for in the presence of 5-fluoroorotic acid (5-FOA). This allows a convenient means of selecting for both forward and backward mutations. The sequence of a 1.8 kb HindIII fragment which contains the functional gene is reported. It encodes a single open reading frame of 264 amino acids which shows considerable conservation with the orotidine-5′-phosphate (OMP) decarboxylases from other organisms. The ura4 transcript is approximately 850 nucleotides long. It begins 51 bp upstream of the protein coding sequence and is unusual in that transcription termination occurs at or very close to the translational stop codon. To facilitate the use of ura4 in gene disruption experiments we have also constructed a novel strain of S. pombe called ura4-D18, in which the 1.8 kbHindIII fragment has been deleted from the chromosome. Using a combination of this strain and vectors containing ura4 as a selectable marker, we present a general method for targeting recombination events to the chromosomal locus under investigation.

Rec8p, a meiotic recombination and sister chromatid cohesion phosphoprotein of the Rad21p family conserved from fission yeast to humans.

ABSTRACT Our work and that of others defined mitosis-specific (Rad21 subfamily) and meiosis-specific (Rec8 subfamily) proteins involved in sister chromatid cohesion in several eukaryotes, including humans. Mutation of the fission yeast Schizosaccharomyces pombe rec8 gene was previously shown to confer a number of meiotic phenotypes, including strong reduction of recombination frequencies in the central region of chromosome III, absence of linear element polymerization, reduced pairing of homologous chromosomes, reduced sister chromatid cohesion, aberrant chromosome segregation, defects in spore formation, and reduced spore viability. Here we extend the description of recombination reduction to the central regions of chromosomes I and II. We show at the protein level that expression ofrec8 is meiosis specific and that Rec8p localizes to approximately 100 foci per prophase nucleus. Rec8p was present in an unphosphorylated form early in meiotic prophase but was phosphorylated prior to meiosis I, as demonstrated by analysis of the mei4mutant blocked before meiosis I. Evidence for the persistence of Rec8p beyond meiosis I was obtained by analysis of the mutantmes1 blocked before meiosis II. A human gene, which we designate hrec8, showed significant primary sequence similarity to rec8 and was mapped to chromosome 14. High mRNA expression of mouse and human rec8 genes was found only in germ line cells, specifically in testes and, interestingly, in spermatids. hrec8 was also expressed at a low level in the thymus. Sequence similarity and testis-specific expression indicate evolutionarily conserved functions of Rec8p in meiosis. Possible roles of Rec8p in the integration of different meiotic events are discussed.

DNA sequence analysis of the ade6 gene of Schizosaccharomyces pombe: Wild-type and mutant alleles including the recombination hot spot allele ade6-M26☆

The gene ade6 is located on chromosome III of the fission yeast Schizosaccharomyces pombe. It codes for the enzyme phosphoribosylaminoimidazole carboxylase involved in purine biosynthesis. A DNA fragment of 3043 nucleotides has been sequenced. It complements ade6 mutations when present on plasmids. An uninterrupted open reading frame of 552 amino acid residues was identified. A method for the cloning of chromosomal mutations by repair of gapped replication vectors in vivo has been developed. Twelve ade6 mutant alleles have been isolated. The sequence alterations of four mutant alleles have been determined. Among them are the ade6-M26 recombination hot spot mutation and the nearby ade6-M375 control mutation. Both are G to T base substitutions, converting adjacent glycine codons to TGA termination codons. They are suppressed by defined tRNA nonsense suppressors of the UGA type. The ade6-M26 mutation leads to a tenfold increase of the occurrence of conversion tetrads in comparison with other ade6 mutations. Possible explanations for the M26-induced increase of recombination frequency are discussed in relation to specific features of the nucleotide sequence identified in the region of the M26 mutation.

Fission yeast Rad50 stimulates sister chromatid recombination and links cohesion with repair

To study the role of Rad50 in the DNA damage response, we cloned and deleted the Schizosaccharo myces pombe RAD50 homologue. The deletion is sensitive to a range of DNA‐damaging agents and shows dynamic epistatic interactions with other recombination–repair genes. We show that Rad50 is necessary for recombinational repair of the DNA lesion at the mating‐type locus and that rad50Δ shows slow DNA replication. We also find that Rad50 is not required for slowing down S phase in response to hydroxy urea or methyl methanesulfonate (MMS) treatment. Interestingly, in rad50Δ cells, the recombination frequency between two homologous chromosomes is increased at the expense of sister chromatid recombination. We propose that Rad50, an SMC‐like protein, promotes the use of the sister chromatid as the template for homologous recombinational repair. In support of this, we found that Rad50 functions in the same pathway for the repair of MMS‐induced damage as Rad21, the homologue of the Saccharomyces cerevisiae Scc1 cohesin protein. We speculate that Rad50 interacts with the cohesin complex during S phase to assist repair and possibly re‐initiation of replication after replication fork collapse.

A specific DNA sequence is required for high frequency of recombination in the ade6 gene of fission yeast.

The point mutation M26 in the ade6 gene of Schizosaccharomyces pombe increases recombination frequency by an order of magnitude in comparison with other mutations in the same gene. The hypothesis is tested that this hot spot of recombination requires a specific nucleotide sequence at the M26 site. The DNA sequence is altered systematically by in vitro mutagenesis, and the resulting sequences are introduced into the ade6 gene in vivo by gene replacement. It results that any change of the heptanucleotide ATGACGT leads to loss of high frequency of recombination. Thus this oligonucleotide sequence is necessary for high frequency of recombination, but it seems not to be sufficient.

Replicating plasmids in Schizosaccharomyces pombe: improvement of symmetric segregation by a new genetic element

We characterized a number of widely used yeast-Escherichia coli shuttle vectors in the fission yeast Schizosaccharomyces pombe. The 2 micron vectors pDB248 and YEp13 showed high frequency of transformation, intermediate mitotic and low meiotic stability, and a low copy number in S. pombe, analogous to their behavior in [cir0] strains of Saccharomyces cerevisiae. The S. cerevisiae integration vectors pLEU2 and pURA3 transformed S. pombe at very low frequencies but, surprisingly, in a nonintegrative fashion. Instead, they replicated autonomously, and they showed very high copy numbers (up to 150 copies per plasmid-containing cell). This could reflect a lack of sequence specificity for replication of plasmid DNA in S. pombe. pFL20, an S. pombe ars vector, and a series of plasmids derived from it were studied to analyze the unusually high stability of this plasmid. Mitotic stability and partitioning of the plasmids was measured by pedigree analysis of transformed S. pombe cells. An S. pombe DNA fragment (stb) was identified that stabilizes pFL20 by improvement of plasmid partitioning in mitosis and meiosis.

Genetic nomenclature and gene list of the fission yeast Schizosaccharomyces pombe

SummaryThe nomenclature rules for the genetics of the fission yeast Schizosaccharomyces pombe have been fixed for the first time, after discussion among scientists working with this organism. Conventions are proposed for the naming of genes and alleles that are obtained by classical means or by reverse genetics. In addition a list has been compiled of 460 known genes of S. pombe. It includes genes defined both by classical mutation analysis and by molecular cloning. 270 genes have been assigned either to one of the three nuclear chromosomes or the mitochondrial genome.

Concerted evolution of tRNA genes: Intergenic conversion among three unlinked serine tRNA genes in S. pombe

In many cases the multiple genes coding for one specific tRNA are dispersed throughout the genome. The members of such a gene family nevertheless maintain a common nucleotide sequence during evolution. A major mechanism contributing to this concerted evolution is intergenic conversion. Here we show that it occurs between three tRNA genes of related sequence residing on different chromosomes of Schizosaccharomyces pombe. Sequence analysis of converted genes indicates that blocks of a minimal length of 18-33 bp and of a maximal length of 190 bp can be transferred from one gene to the other. During meiosis the frequency of these transfers lies in the order of 10(-5) per progeny spore. Information transfer between any two members of the gene family occurs in both directions.

The strong ADH1 promoter stimulates mitotic and meiotic recombination at the ADE6 gene of Schizosaccharomyces pombe

The effect of the strong promoter from the alcohol dehydrogenase gene on mitotic and meiotic intragenic recombination has been studied at the ade6 locus of the fission yeast Schizosaccharomyces pombe. A 700-bp fragment containing the functional adh1 promoter was used to replace the weak wild-type promoter of the ade6 gene. Analysis of mRNA showed that strains with this ade6::adh1 fusion construct had strongly elevated ade6-specific mRNA levels during vegetative growth as well as in meiosis. These increased levels of mRNA correlated with a 20- to 25-fold stimulation of intragenic recombination in meiosis and a 7-fold increased prototroph formation during vegetative growth. Analysis of flanking marker configurations of prototrophic recombinants indicated that simple conversions as well as conversions associated with crossing over were stimulated in meiosis. The strongest stimulation of recombination was observed when the adh1 promoter was homozygous. Studies with heterologous promoter configurations revealed that the highly transcribed allele was the preferred acceptor of genetic information. The effect of the recombinational hot spot mutation ade6-M26 was also investigated in this system. Its effect was only partly additive to the elevated recombination rate generated by the ade6::adh1 fusion construct.

MICROTUBULE-DRIVEN NUCLEAR-MOVEMENTS AND LINEAR ELEMENTS AS MEIOSIS-SPECIFIC CHARACTERISTICS OF THE FISSION YEASTS SCHIZOSACCHAROMYCES-VERSATILIS AND SCHIZOSACCHAROMYCES-POMBE

Meiotic prophase in Schizosaccharomyces pombe is characterized by striking nuclear movements and the formation of linear elements along chromosomes instead of tripartite synaptonemal complexes. We analysed the organization of nuclei and microtubules in cells of fission yeasts undergoing sexual differentiation. S. japonicus var. versatilis and S. pombe cells were studied in parallel, taking advantage of the better cytology in S. versatilis. During conjugation, microtubules were directed towards the mating projection. These microtubules seem to lead the haploid nuclei together in the zygote by interaction with the spindle pole bodies at the nuclear periphery. After karyogamy, arrays of microtubules emanating from the spindle pole body of the diploid nucleus extended to both cell poles. The same differentiated microtubule configuration was elaborated upon induction of azygotic meiosis in S. pombe. The cyclic movements of the elongated nuclei between the cell poles is reflected by a dynamic and coordinated shortening and lengthening of the two microtubule arrays. When the nucleus was at a cell end, one array was short while the other bridged the whole cell length. Experiments with inhibitors showed that microtubules are required for karyogamy and for the elongated shape and movement of nuclei during meiotic prophase. In both fission yeasts the SPBs and nucleoli are at the leading ends of the moving nuclei. Astral and cytoplasmic microtubules were also prominent during meiotic divisions and sporulation. We further show that in S. versatilis the linear elements formed during meiotic prophase are similar to those in S. pombe. Tripartite synaptonemal complexes were never detected. Taken together, these findings suggest that S. pombe and S. versatilis share basic characteristics in the organization of microtubules and the structure and behaviour of nuclei during their meiotic cell cycle. The prominent differentiations of microtubules and nuclei may be involved in the pairing, recombination, and segregation of meiotic chromosomes.