The effect of actidione on mitosis in the slime mold Physarum polycephalum

Regulation of tubulin synthesis during the cell cycle in the synchronous plasmodia of Physarum polycephalum

Regulation of alpha- and beta-tubulin isotype synthesis during the cell cycle has been studied in the myxomycete Physarum polycephalum, by subjecting synchronous plasmodia to temperature shifts and pharmacological perturbations. Temperature shifts interfered with the regulation of tubulin synthesis. Inhibition of DNA synthesis prevents tubulin degradation after completion of the cell cycle (Ducommun and Wright, Eur. J. Cell Biol., 50:48-55, 1989) but did not perturb the initiation of tubulin synthesis. The constant increase of tubulin synthesis in the presence of tubulin-sequestering drugs and the decrease of tubulin synthesis during a treatment with aphidicolin in late G2 phase suggest the existence of an autoregulatory mechanism of tubulin synthesis. Moreover, the microtubule poison methyl benzimidazole carbamate dissociated synthesis of the alpha 1-tubulin isotype from the generally strictly coordinated synthesis of all tubulin isotypes during the transient interruption of mitosis. These observations show that a microtubular poison can perturb regulation of the synthesis of specific isotubulins.

Anion-Sensitive Mg ATP-Dependent Proton Pumping in Microsomal Membranes from Phycomyces blakesleeanus Bgff

A light microsomal membrane fraction (collected on 28% weight/volume sucrose) from young mycelia of Phycomyces blakesleeanus Bgff. stationary cultures exhibited Mg(2+) ATP-dependent proton pumping as monitored with the dye neutral red or by accumulation of [(14)C]methylamine. The substrate kinetics for ATP (K(m(ATP)) 1.1 millimolar in the presence of 2 millimolar Mg(2+)), the competitive inhibition by ADP (K(i(ADP)) 0.8 millimolar), the anion sensitivity (stimulation by Cl(-), inhibition by NO(3) (-) and SO(4) (2-)), the reversal of acidification by Ca(2+), the inhibition by diethylstilbestrol and dicyclohexylcarbodiimide, as well as the absence of any inhibition by azide, molybdate, and vanadate strongly suggest a tonoplast-type ATPase driven proton pumping. The same membrane fraction showed no pyrophosphate driven H(+)-translocation. Electron microscopic examination of the fungal mycelium demonstrated the presence of vacuoles of different sizes and other nonidentified vesicles but no typical Golgi stacks.

Induction of polyploid nuclei in Physarum polycephalum by cycloheximide treatment in prophase

Macroplasmodia of the acellular slime mold Physarum polycephalum were treated with pulses of cycloheximide (10 micrograms/ml medium, for 3 h), initiated 10-20 min before metaphase in the synchronous nuclear division cycle. This treatment interfered with normal division of the nuclei, but permitted DNA synthesis in the next S phase. This interpretation is supported by measurements of the DNA content per nucleus in cycloheximide-treated cultures as compared to control cultures, which show that some nuclei after cycloheximide treatment are polyploid. By this method we can produce polyploid strains of Physarum, but the elevated nuclear DNA content is not stable, and after several months the strains have reverted to the normal diploid DNA content.

Translational regulation of polysome formation during dormancy of Physarum polycephalum

The translational activity of actively growing microplasmodia and dormant microsclerotia of Physarum polycephalum was investigated by analyzing the distribution of ribosomes in polysomes. Microplasmodial post-mitochondrial fractions contained substantial amounts of polysomes and ribosomal subunits but very few native monosomes. During the starvation period which preceded microsclerotium formation, polysome levels remained constant, whereas the subunit titer began to increase. During encystment ribosomal subunits continued to accumulate as the level of polysomes gradually decreased. Dormant microsclerotia contained a large surplus of stored ribosomal subunits but no detectable polysomes. However, incubation of microsclerotia with concentrations of cycloheximide sufficient to slow polypeptide elongation without affecting initiation caused the gradual reappearance of polysomes at the expense of the subunits. Under these conditions the percentage of subunits driven into polysomes reached values similar to those of actively growing microplasmodia. Microsclerotia returned to nutrient medium contained very low levels of polysomes during the lag period which preceded germination. These were formed with preexisting, stored messenger ribonucleic acid. During the germination period, polysome levels were markedly increased. This elevation was dependent on new ribonucleic acid transcription. It is concluded that dormant microsclerotia contain functional messenger ribonucleic acid and ribosomes which are subject to translational repression at the level of initiation.

An immunological approach to the isolation of factors with mitotic activity from the plasmodial stage of the myxomycete Physarum polycephalum

Nuclear divisions in plasmodia of Physarum polycephalum were advanced by applying immunologically purified plasmodial extracts of late G2 phase on the surface of plasmodia which were 1.5 h before the third mitosis. The purification of G2 extracts was achieved by interaction of antibodies prepared against the antigens of early S phase plasmodia with the antigens of late G2 plasmodia. There was no advancement of mitosis by extracts prepared from early S phase plasmodia. Untreated G2 extracts did not accelerate mitosis with the same effectiveness as did antibody purified G2 extracts.

Cycloheximide-induced mitotic delay in Physarum polycephalum

Cycloheximide pulses applied to Physarum polycephalum surface plasmodia delay mitosis. Pulses applied in G2 cause a delay of mitosis which is linearly dependent on the phase in the cell cycle at which the pulse is applied. A 30 min pulse of 10 micrograms/ml cycloheximide starting in G2 at time t after mitosis induces an excess delay (delay in excess of pulse duration) of the next mitosis of (0.55) t-1.3 h. The excess delays induced by 7 h pulses during G2 are at most 1 h larger. Pulses applied less than 30 min before mitosis induce only small delays.

Topical cycloheximide in psoriasis

Cycloheximide is an antimitotic and an inhibitor of the synthesis of both DNA and protein. Topical application in low concentrations (0.1%-0.2%) resulted in definite involution of psoriatic plaques. Because of its potency this experimental drug - not yet available for clinical use - was employed on small areas only and with extreme caution. Results of a full evaluation for safety, currently in progress, are being awaited with interest.

[Modern research on the cell cycle and mitosis in the myxomycete Physarum polycephalum]

The plasmodial stage of Physarum polycephalum contains up to 10(9) nuclei which undergo a naturally synchronous mitosis every 8 h. Nuclear processes such as DNA and RNA synthesis as well as many cytoplasmic processes such as histone synthesis are also synchronous. Physarum polycephalum is therefore widely used in studies of cell-cycle events. This article describes experiments that may help to explain two fundamental biological processes: (1) the mechanism that triggers mitosis, (2) the structural basis of mitotic movement.

Uridine diphosphoglucose pyrophosphorylase activity and differentiation in the cellular slime mold Physarum polycephaluno

The specific activity of uridine 5'-triphosphate:alpha-d-glucose 1-phosphate uridyltransferase (EC 2.7.7.9) (also called uridine 5'-diphosphate [UDP]-glucose pyrophosphorylase) has been found to increase up to eightfold during spherule formation by the slime mold Physarum polycephalum. The enzyme accumulates during the first 8 to 9 h after initiation of spherule formation, declines to basal levels found in vegetative microplasmodia by 15 h, and is undetectable in completed spherules. Specific activities for UDP-glucose pyrophosphorylase in vegetative microplasmodia range from 15 to 30 nmol of UDP-glucose formed per min per mg of protein, whereas accumulated levels during spherule formation can attain a specific activity as high as 125 nmol of UDP-glucose formed per min per mg of protein. The scheduling and extent of accumulation are critically dependent on an early log-phase age of microplasmodia originally induced to form spherules. Spherule induction by 0.2 M or 0.5 M mannitol delays this schedule in a variable and unpredictable manner. Spherule-forming microplasmodia which have accumulated high levels of UDP-glucose pyrophosphorylase spontaneously excrete the enzyme when transferred to salts medium containing 0.2 M or 0.5 M mannitol. The excreted enzyme is subsequently destroyed or inactivated. Studies with preferential inhibitors of macromolecular synthesis indicate that accumulation of UDP-glucose pyrophosphorylase requires concomitant protein synthesis and prior ribonucleic acid synthesis.

Activity of uridine diphosphate N-acetylglucosamine-4-epimerase during spherulation of Physarum polycephalum

The specific activity of uridine diphosphate N-acetylglucosamine-4-epimerase increases during spherulation of Physarum polycephalum, a process that involves the synthesis of galactosamine walls. This increase is prevented by the addition of cycloheximide.

Effect of cycloheximide on the cell cycle of the crypts of the small intestine of the rat

A single injection of 1.5 mg/kg of cycloheximide induces a complete disappearance of mitotic activity in rat intestinal crypts within 1.5-2 hr. No significant necrosis of crypt cells is observed even though this phenomenon is accompanied by a marked decrease in uptake of labeled precursors into protein and DNA. Mitoses reappear 6 hr after injection and recovery then follows a cyclic pattern over a period equivalent to one cell cycle, thereby reflecting at least a partial synchronization of cell division. Concurrent use of colchicine, an agent known to induce metaphase arrest, has demonstrated that cycloheximide, while having no apparent effect on cells already in division, prevents the entrance of new cells into visible mitosis. Analysis of the cell cycle suggests that one block initiated by cycloheximide occurs in G(2), presumably as the result of an interference with the formation of protein(s) required for the normal progression of cells from this phase of the cycle into mitosis.

Studies on the maintenance of oral development in Tetrahymena pyriformis GL-C. II. The relationship of protein synthesis to cell division and oral organelle development

The effects of puromycin on synchronized Tetrahymena pyriformis were investigated at two different concentrations, 43 microg per ml and 430 microg per ml. The rate of incorporation of histidine-(14)C into hot TCA-insoluble material was reduced by 30% at the low concentration and by 80-90% at the high concentration. The rate of oxygen uptake was lowered by only 10-20% at both concentrations. Cell division was prevented at both concentrations, if the drug was added prior to a "transition point" at about 45 min after the end of the synchronizing treatment. Development of "anarchic field" oral primordia was arrested, while primordia in early stages of membranelle differentiation were resorbed. Resorption began shortly after addition of the drug, and proceeded most rapidly at the lower concentration. If the drug was added after the "transition point," cell division and oral primordium formation were completed with only slight delay at the low concentration, and with considerable delay (in some cases complete arrest) at the high concentration. The results thus indicate that protein synthesis is involved in the later as well as the earlier stages of development; what specially characterizes the earlier stages, prior to the "transition point," is a dramatic response to partial inhibition of protein synthesis. It is suggested that this response involves the activation or release of a latent intracellular degradative system which is specific for developing structures.

Anaphase delay after inhibition of protein synthesis between late prophase and prometaphase

The relationship between protein synthesis and mitosis was studied in Physarum polycephalum, a plasmodial slime mold whose nuclear divisions are synchronous. Results of studies with actidione (cycloheximide), an inhibitor of protein synthesis, indicated that the essential structural proteins for mitosis and nuclear reconstruction were completed prior to the dissolution of the nucleolus in prophase. Proteins that determine the duration of the transition from metaphase to nuclear reconstruction were synthesized from late prophase to prometaphase. It is proposed that these proteins are concerned with the transformation of chemical energy into the mechanical work of mitosis.

Limited DNA synthesis in the absence of protein synthesis in Physarum polycephalum

Actidione (cycloheximide), an antibiotic inhibitor of protein synthesis, blocked the incorporation of leucine and lysine during the S phase of Physarum polycephalum. Actidione added during the early prophase period in which mitosis is blocked totally inhibited the initiation of DNA synthesis. Actidione treatment in late prophase, which permitted mitosis in the absence of protein synthesis, permitted initiation of a round of DNA replication making up between 20 and 30% of the unreplicated nuclear DNA. Actidione treatment during the S phase permitted a round of replication similar to the effect at the beginning of S. The DNA synthesized in the presence of actidione was replicated semiconservatively and was stable through at least the mitosis following antibiotic removal. Experiments in which fluorodeoxyuridine inhibition was followed by thymidine reversal in the presence of actidione suggest that the early rounds of DNA replication must be completed before later rounds are initiated.

Genetic analysis of actidione-resistant mutants in the Myxomycete Physarum polycephalum, Schw

1. Improved methods for the culture and preservation ofPhysarum polycephalumamoebae have been developed.

2. Strains of amoebae have been isolated which are stably resistant to 4, 8 or 16 μg./ml. actidione.

3. Sensitivity vs. resistance to 4 μg./ml. actidione is probably controlled by a single pair of alleles at a locus unlinked to the mating-type locus.

4. Plasmodia homozygous or heterozygous for the allele determining resistance at this locus are no more resistant to actidione than plasmodia homozygous for the allele determining sensitivity.