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Herbert Tabor

Publications and source records attributed to Herbert Tabor.

10 recordsLinked to original sources
PubMed2006

Polyamine deficiency leads to accumulation of reactive oxygen species in a spe2Delta mutant of Saccharomyces cerevisiae.

We have previously shown that polyamine-deficient Saccharomyces cerevisiae are very sensitive to incubation in oxygen. The current studies show that, even under more physiological conditions (i.e. growth in air), polyamine-deficient cells accumulate reactive oxygen species (ROS). These cells develop an apoptotic phenotype and, after incubation in polyamine-deficient medium, die. To show a specific effect of polyamines on ROS accumulation, uncomplicated by any effects on growth, spermine was added to spermidine-deficient spe2Delta fms1Delta cells, since spermine does not affect the growth of this strain. In this strain, spermine addition caused a marked, but not complete, decrease in the accumulation of ROS and a moderate protection against cell death. In other experiments with polyamine-deficient cells containing plasmids that overexpress superoxide dismutases (SOD1, SOD2), ROS decreased but with only a partial protection against cell death. Polyamine-deficient cells incubated anaerobically show markedly less cell death. These data show that part of the function of polyamines is protection of the cells from accumulation of ROS.

Apoptosis↗
PubMed2006

Methylthioadenosine and polyamine biosynthesis in a Saccharomyces cerevisiae meu1delta mutant.

As part of our studies on polyamine biosynthesis in yeast, the metabolism of methylthioadenosine was studied in a mutant that lacks methylthioadenosine phosphorylase (meu1delta). The nucleoside accumulates in this mutant and is mainly excreted into the culture medium. Intracellular accumulation of the nucleoside is enough to account for the inhibition of spermidine synthase and thus to indirectly regulate the polyamine content of the meu1delta cells. By comparing the results with this mutant with a meu1delta spe2delta mutant that cannot synthesize spermidine or spermine, we showed that >98% of methylthioadenosine is produced as a byproduct of polyamine synthesis (i.e., from decarboxylated S-adenosylmethionine). In contrast, in MEU1+ SPE2+ cells methylthioadenosine does not accumulate and is metabolized through the methionine salvage pathway. Using a met15delta mutant we show that this pathway (i.e., involving polyamine biosynthesis and methylthioadenosine metabolism) is a significant factor in the metabolism of methionine, accounting for 15% of the added methionine.

Adenosine↗
PubMed2005

Studies on the regulation of ornithine decarboxylase in yeast: effect of deletion in the MEU1 gene.

Methylthioadenosine is formed during the biosynthesis of spermidine and of spermine and is metabolized by methylthioadenosine phosphorylase, an enzyme missing in several tumor cell lines. In Saccharomyces cerevisiae, this enzyme is coded by the MEU1 gene. We have now studied the effect of the meu1 deletion on polyamine metabolism in yeast. We found that the effects of the meu1Delta mutation mostly depend on the stage of cell growth. As the cell density increases, there is a marked fall in the level of ornithine decarboxylase (ODC) in the MEU1(+) cells, which we show is caused by an antizyme-requiring degradation system. In contrast, there is only a small decrease in the ODC level in the meu1Delta cells. The meu1Delta cells have a higher putrescine and a lower spermidine level than MEU1(+) cells, suggesting that the decreased spermidine level in the meu1Delta cultures is responsible for the greater apparent stability of ODC in the meu1Delta cells. The lower spermidine level in the meu1Delta cells probably results from an inhibition of spermidine synthase by the methylthioadenosine that presumably accumulates in these mutants. In both MEU1(+) and the meu1Delta cultures, the ODC levels were markedly decreased by the addition of spermidine to the media, and thus our results contradict the postulation of Subhi et al. [Subhi, A. L., et al. (2003) J. Biol. Chem. 278, 49868-49873] of a novel regulatory pathway in meu1Delta cells in which ODC is not responsive to spermidine.

Methionine↗
PubMed2003

Spermidine but not spermine is essential for hypusine biosynthesis and growth in Saccharomyces cerevisiae: spermine is converted to spermidine in vivo by the FMS1-amine oxidase.

In our earlier work we showed that either spermidine or spermine could support the growth of spe2Delta or spe3Delta polyamine-requiring mutants, but it was unclear whether the cells had a specific requirement for either of these amines. In the current work, we demonstrate that spermidine is specifically required for the growth of Saccharomyces cerevisiae. We were able to show this specificity by using a spe3Delta fms1Delta mutant that lacked both spermidine synthase and the FMS1-encoded amine oxidase that oxidizes spermine to spermidine. The polyamine requirement for the growth of this double mutant could only be satisfied by spermidine; i.e., spermine was not effective because it cannot be oxidized to spermidine in the absence of the FMS1 gene. We also showed that at least one of the reasons for the absolute requirement for spermidine for growth is the specificity of its function as a necessary substrate for the hypusine modification of eIF5A. Spermine itself cannot be used for the hypusine modification, unless it is oxidized to spermidine by the Fms1 amine oxidase. We have quantified the conversion of spermine in vivo and have shown that this conversion is markedly increased in a strain overexpressing the Fms1 protein. We have also shown this conversion in enzymatic studies by using the purified amine oxidase from yeast.

Base Sequence↗
PubMed2003

Polyamines protect Escherichia coli cells from the toxic effect of oxygen.

Wild-type Escherichia coli cells grow normally in 95% O(2)/5% CO(2). In contrast, cells that cannot make polyamines because of mutations in the biosynthetic pathway are rapidly killed by incubation in 95% O(2)/5% CO(2). Addition of polyamines prevents the toxic effect of oxygen, permitting cell survival and optimal growth. Oxygen toxicity can also be prevented if the growth medium contains an amino acid mixture or if the polyamine-deficient cells contain a manganese-superoxide dismutase (Mn-SOD) plasmid. Partial protection is afforded by the addition of 0.4 M sucrose or 0.4 M sorbitol to the growth medium. We also report that concentrations of H(2)O(2) that are nontoxic to wild-type cells or to mutant cells pretreated with polyamines kill polyamine-deficient cells. These results show that polyamines are important in protecting cells from the toxic effects of oxygen.

Air↗
PubMed2002

Absolute requirement of spermidine for growth and cell cycle progression of fission yeast (Schizosaccharomyces pombe).

Schizosaccharomyces pombe cells that cannot synthesize spermidine or spermine because of a deletion-insertion in the gene coding for S-adenosylmethionine decarboxylase (Deltaspe2) have an absolute requirement for spermidine for growth. Flow cytometry studies show that in the absence of spermidine an overall delay of the cell cycle progression occurs with some accumulation of cells in the G(1) phase; as little as 10(-6) M spermidine is sufficient to maintain normal cell cycle distribution and normal growth. Morphologically some of the spermidine-deprived cells become spherical at an early stage with little evidence of cell division. On further incubation in the spermidine-deprived medium, growth occurs in most of the cells, not by cell division but rather by cell elongation, with an abnormal distribution of the actin cytoskeleton, DNA (4', 6-diamidino-2-phenylindole staining), and calcofluor-staining moieties. More prolonged incubation in the spermidine-deficient medium leads to profound morphological changes including nuclear degeneration.

Adenosylmethionine Decarboxylase↗