F. Manna

A mitochondrial group-I intron in fission yeast encodes a maturase and is mobile in crosses.

The open reading frame in the first intron of the mitochondrial gene encoding subunit I of cytochrome c oxidase encodes a maturase and stimulates homologous recombination in Escherichia coli. In this paper, we demonstrate that this intron is mobile in crosses, indicating that it also encodes an endonuclease. This is the first report on an intron which possesses mobility and acts as a maturase.

Complete absence of mitochondrial DNA in the petite-negative yeast Schizosaccharomyces pombe leads to resistance towards the alkaloid lycorine.

The petite-positive yeastSaccharomyces cerevisiae can be efficiently and completely converted to respiratory-deficient cytoplasmic petite mutants by intercalating drugs.Rhoo petites fromSchizosaccharomyces pombe could only be obtained in strains carrying a nuclear mutation. In this paper we report the efficient isolation ofrhoo mutants in aSch. pombe strain containing a mitochondrial mutator mutation. We also show that the alkaloid lycorine is able to differentiate between cells containing defective mitochondrial DNA (mit−) and those lacking mitochondrial DNA completely (rhoo).Rhoo cells are resistant to the alkaloid whereasmit− and wild-type cells show the same sensitivity.

Differential effect of the alkaloid lycorine on rho (+), mit (-), rho (-), and rho (o) strains of Saccharomyces cerevisiae.

SummaryThe alkaloid lycorine inhibits growth of rho+, mit−, and rho− strains, whereas rho0 strains (devoid of mitochondrial DNA) of Saccharomyces cerevisiae are resistant to high concentrations of the drug. The inhibitory effect of lycorine on the growth of rho+ strains can be counteracted by the addition of ascorbic acid to the plating medium. Lycorine induces cytoplasmic respiratory deficient mutants upon growth in glucose complete medium containing the alkaloid. The minimal inhibitory concentration of lycorine in glucose complete medium is ten times higher than in glycerol complete medium, thus demonstrating its preferential action on mitochondrial functions. Total protein synthesis and mitochondrial protein synthesis are only slightly inhibited by lycorine. It has, however, an inhibitory effect on DNA and RNA synthesis.

Interactions between the yeast mitochondrial and nuclear genomes: isogenic suppressive and hypersuppressive petites differ in their resistance to the alkaloid lycorine

SummaryIn a previous paper we have shown that the alkaloid lycorine inhibits growth of rho+, mit- and rho-, strains of Saccharomyces cerevisiae, whereas strains devoid of mitochondrial DNA (rhoo) are resistant to more than 200 μg/ml of the alkaloid. In this report we show that hypersuppressive petites are almost as resistant as rhoo mutants, whereas isogenic rho- petites, which have retained tained longer segments of the genome, are sensitive to the drug.

The mitochondrial genome of Schizosaccharomyces pombe

SummaryThe open reading frame of the first intron of the mitochondrial cox1 gene (cox1I1) was expressed in Escherichia coli. The putative intron-encoded protein stimulated the formation of intra-chromosomal lac+-recombinants about threefold. No stimulation was found when the reading frame was inserted in the opposite direction, or when it was interrupted by a deletion. The intronic open reading frame did not complement recA− or recB− mutants of E. coli. In S. pombe, elimination of this intron did not abolish homologous recombination in mitochondria. A possible role of the recombinase activity in yeast mitochondria will be discussed.

News & Notes: Correlation of Resistance to the Alkaloid Lycorine with the Degree of Suppressiveness in Petite Mutants of Saccharomyces cerevisiae

Abstract. In previous papers (Del Giudice et al. Curr Genet 8:493–497, 1984; Massardo et al. Curr Genet 17:455–457, 1985) we have shown that strains of Saccharomyces cerevisiae that are devoid of mitochondrial DNA (rhoo) are resistant to the alkaloid lycorine isolated from Amaryllis plants, whereas strains containing mitochondrial DNA (rho−, mit−, or rho+) are sensitive to this drug. In addition, we were able to show that the so-called hypersuppressive petites, whose mitochondrial genomes consist of short regions of DNA containing an ori sequence, show intermediate resistance. In this paper, we demonstrate that the degree of suppressiveness of a rho− mutant correlates with the degree of resistance to lycorine.

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

SummaryIn 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.

Two Extramitochondrial Circular DNA Species in the Petite Negative Yeast Schizosaccharomyces pombe: Relative abundance and size determination by electron microscopy.

SummaryIn this paper we present the electron microscopic analysis of two distinct extramitochondrial circular DNA species in the fission yeast Schizosaccharomyces pombe (S. pombe). Both DNA species can be isolated from mitochondrial fractions, but disappear after DNase treatment of mitochondria, demonstrating their extramitochondrial location. The size of these molecular species is 3.08 ± 0.18 μm and 2.00 ± 0.09 μm (standard deviation). They are present in a ratio of approximately 9:1 in the DNA preparations analyzed.

Isolation and genetical and biochemical characterization of mutants resistant to the alkaloid lycorine

SummaryMutants resistant to 200 µg/ml of the alkaloid lycorine (LYCR) in non-fermentable substrate were isolated after nitrosoguanidine mutagenesis. Tetrad analysis and growth of heterozygous (LYCR/lycs) diploids from two different mutants revealed that a single nuclear and dominant mutation is responsible for the resistant phenotype. In the wild type total protein synthesis is only slightly inhibited, whereas DNA and RNA synthesis is lowered to about 30% of the control. In the lycorine resistant mutants all macromolecular syntheses are unaffected by the drug.

Expression of cloned mitochondrial DNA from the petite negative yeast Schizosaccharomyces pombe in E. coli minicells

SummaryThe minicell producing Escherichia coli strain D24 (lysogenic for phage lambda cI857) was transformed with the recombinant plasmid pDG3 containing the entire mitochondrial (mt) genome of the fission yeast Schizosaccharomyces pombe (S. pombe) cloned in the single BamHI-site of the E. coli plasmid pBR322 (Del Giudice 1981). By DNA-RNA hybridization it could be shown that the total mtDNA sequence of the plasmid pDG3 was transcribed in the E. coli minicells. The cloned mtDNA also directed the synthesis of at least five novel polypeptides with molecular weights between 7,200 and 34,000. When the minicell producing E. coli strain P678-54 was transformed with the hybrid plasmid pDG3, considerable portions of the inserted mtDNA sequences were deleted. One of the resulting plasmids (pDG4), lacking about two-thirds of the mtDNA sequence, directed the synthesis of new polypeptides in the range of 7,000 to 17,500 daltons. Another derivative of pDG3, the plasmid pDG5, containing one-sixth of the mtDNA sequence, directed the synthesis of at least three novel polypeptides. The mt origin of novel polypeptides coded by the hybrid plasmid pDG3 was demonstrated by use of antisera raised against total mitochondrial proteins from S. pombe and antisera against subunits II and III of cytochrome c oxidase from Saccharomyces cerevisiae (S.cer.).