Fluctuations in deoxyribo- and ribonucleoside triphosphate pools during the mitotic cycle of Physarum polycephalum

Correlation between tubulin mRNA stability and poly(A) length over the cell cycle of Physarum polycephalum

During the cell cycle of the Physarum polycephalum plasmodium, levels of alpha-tubulin mRNA rise exponentially in G2 phase, reach a peak at metaphase 40-fold above basal levels, and then fall exponentially to basal levels after mitosis. We show that post-mitotic alpha-tubulin mRNA carries poly(A) tracts of less than 30 residues. By contrast, when levels of alpha-tubulin mRNA rise during G2 phase, the mRNA has a poly(A) tract of approximately 80 bases. The length of the poly(A) tract of any mRNA encoding actin is relatively constant at fewer than 30 bases through the cycle. We have estimated the apparent rate of synthesis of alpha-tubulin mRNA at different stages of the cell cycle by short-term labeling in vivo. Transcription of alpha-tubulin mRNA continues even after mitosis, though the rate may be diminished relative to that in late G2 phase. So, the post-mitotic molecular half-life of alpha-tubulin mRNA must be less than the 19 minute half-life by which the levels of this species fall. The fact that the apparent rate of alpha-tubulin mRNA synthesis is not vastly greater in early G2 phase than in post-mitotic plasmodia is consistent with an S-phase destabilization of alpha-tubulin mRNA molecules. Thus, the poly(A) tail is shorter when the alpha-tubulin mRNA is less stable.

In vivo levels of diadenosine tetraphosphate and adenosine tetraphospho-guanosine in Physarum polycephalum during the cell cycle and oxidative stress

Cellular levels of diadenosine tetraphosphate (Ap4A) and adenosine tetraphospho-guanosine (Ap4G) were specifically measured during the cell cycle of Physarum polycephalum by a high-pressure liquid chromatographic method. Ap4A was also measured indirectly by a coupled phosphodiesterase-luciferase assay. No cell cycle-specific changes in either Ap4A or Ap4G were detected in experiments involving different methods of assay, different strains of P. polycephalum, or different methods of fixation of macroplasmodia. Our results on Ap4A are in contrast with those reported previously (C. Weinmann-Dorsch, G. Pierron, R. Wick, H. Sauer, and F. Grummt, Exp. Cell Res. 155:171-177, 1984). Weinmann-Dorsch et al. reported an 8- to 30-fold increase in Ap4A in early S phase in P. polycephalum, as measured by the phosphodiesterase-luciferase assay. We also measured levels of Ap4A, Ap4G, and ATP in macroplasmodia treated with 0.1 mM dinitrophenol. Ap4A and Ap4G transiently increased three- to sevenfold after 1 h and then decreased concomitantly with an 80% decrease in the level of ATP after 2 h in the presence of dinitrophenol. These results do not support the hypothesis that Ap4A is a positive pleiotypic activator that modulates DNA replication, but they are consistent with the hypothesis proposed for procaryotes that Ap4A and Ap4G are signal nucleotides or alarmones of oxidative stress (B.R. Bochner, P.C. Lee, S.W. Wilson, C.W. Cutler, and B.N. Ames, Cell 37:225-232, 1984).

In vivo levels of diadenosine tetraphosphate and adenosine tetraphospho-guanosine in Physarum polycephalum during the cell cycle and oxidative stress

Cellular levels of diadenosine tetraphosphate (Ap4A) and adenosine tetraphospho-guanosine (Ap4G) were specifically measured during the cell cycle of Physarum polycephalum by a high-pressure liquid chromatographic method. Ap4A was also measured indirectly by a coupled phosphodiesterase-luciferase assay. No cell cycle-specific changes in either Ap4A or Ap4G were detected in experiments involving different methods of assay, different strains of P. polycephalum, or different methods of fixation of macroplasmodia. Our results on Ap4A are in contrast with those reported previously (C. Weinmann-Dorsch, G. Pierron, R. Wick, H. Sauer, and F. Grummt, Exp. Cell Res. 155:171-177, 1984). Weinmann-Dorsch et al. reported an 8- to 30-fold increase in Ap4A in early S phase in P. polycephalum, as measured by the phosphodiesterase-luciferase assay. We also measured levels of Ap4A, Ap4G, and ATP in macroplasmodia treated with 0.1 mM dinitrophenol. Ap4A and Ap4G transiently increased three- to sevenfold after 1 h and then decreased concomitantly with an 80% decrease in the level of ATP after 2 h in the presence of dinitrophenol. These results do not support the hypothesis that Ap4A is a positive pleiotypic activator that modulates DNA replication, but they are consistent with the hypothesis proposed for procaryotes that Ap4A and Ap4G are signal nucleotides or alarmones of oxidative stress (B.R. Bochner, P.C. Lee, S.W. Wilson, C.W. Cutler, and B.N. Ames, Cell 37:225-232, 1984).

Fluctuations in S-adenosyl-L-methionine (AdoMet) during mitotic cycle of the myxomycete Physarum polycephalum

A sensitive radioisotope dilution method was used to measure the S-adenosyl-L-methionine (AdoMet) content in macroplasmodia of the slime mold Physarum polycephalum during the mitotic cycle. The AdoMet pool had two maxima, one during mitosis, the other in the middle of G2 phase.

Organization of DNA replication in Physarum polycephalum. Attachment of origins of replicons and replication forks to the nuclear matrix

We have investigated the attachment of the DNA to the nuclear matrix during the division cycle of the plasmodial slime mold Physarum polycephalum. The DNA of plasmodia was pulse labelled at different times during the S phase and the label distribution was studied by graded DNase digestion of the matrix-DNA complexes prepared from nuclei isolated by extraction with 2 M NaCl. Pulse labelled DNA was preferentially recovered from the matrix bound residual DNA at any time of the S phase. Label incorporated at the onset of the S phase remained preferentially associated with the matrix during the G2 phase and the subsequent S phase. The occurrence of the pulse label in the matrix associated DNA regions was transiently elevated at the onset of the subsequent S phase. Label incorporated at the end of the S phase was located at DNA regions which, in the G2 phase, were preferentially released from the matrix by DNase treatment. From the results and previously reported data on the distribution of attachment sites it can be concluded that origins of replicons or DNA sites very close to them are attached to the matrix during the entire nuclear cycle. The data further indicate that initiations of DNA replication occur at the same origins in successive S phases. Replicating DNA is bound to the matrix, in addition, by the replication fork or a region close to it. This binding is loosened after completion of the replication.

Purine metabolism in microplasmodia of Physarum polycephalum

The uptake and utilization of purine nucleosides and purines in microplasmodia of Physarum polycephalum were investigated. The results revealed a unique pattern, namely that exogenous purine nucleosides are readily taken up and metabolised, while free purine bases are hardly taken up. The pathways of incorporation have been elucidated in studies with whole cells and with cell-free extracts. The ribonucleosides (adenosine, inosine and guanosine) can be converted into ribonucleotides in two ways; either directly catalysed by a kinase or by a phosphorolytic cleavage to the free base (adenine, hypoxanthine and guanine respectively) which can then be activated by a purine phosphoribosyltransferase. Apparently the purine phosphoribosyltransferases do not react with exogenous purine bases. The deoxyribonucleosides (deoxyadenosine, deoxyinosine and deoxyguanosine) are also phosphorolysed by purine nucleoside phosphorylase to adenine, hypoxanthine and guanine respectively. A portion of deoxyadenosine is directly phosphorylated to dAMP. It appears that only a minor part of the soluble nucleotide pool can be synthesised from exogenous supplied nucleosides and that none of the deoxyribonucleosides specifically label DNA. There is no catabolism of the purine moiety. In agreement with the above findings, we have found that analoguees of purine nucleosides are more toxic than their corresponding purine base analogues.

Pyrimidine metabolism in microplasmodia of Physarum polycephalum

If microplasmodia of Physarum polycephalum are exposed to 14C-labelled pyrimidine nucleosides or bases, an unusual pattern of metabolism is found. Only the nucleosides are taken up. Analysis of the distribution of the radioactivity in the cells revealed that ribonucleosides and deoxyribonucleosides are incorporated into nucleotides; however, a substantial catabolism takes place. Thus incubation with [2-14C]pyrimidine nucleosides readily gives rise to [14C]O2, particularly in the case of [2-14C]thymidine. Due to this a significant part of the trichloroacetic-acid-insoluble radioactivity from exogenously supplied [2-14C]thymidine is not associated with DNA. The pattern of labelling of nucleoside triphosphates from exogenously supplied nucleosides indicated that the de novo synthesis of nucleotides was only partly repressed. An unusual conversion of deoxycytidine into cytidine was noted. Enzyme analysis on cell-free extracts revealed that pyrimidine nucleosides can be salvaged by kinases and that their initial catabolism is initiated by hydrolases. Incubation of microplasmodia with pyrimidine analogues showed that only nucleoside analogues are toxic. The experimental results have led us to propose a scheme for the metabolism of pyrimidine nucleosides and bases in Physarum polycephalum.

Physarum action. Observations on its presence, stability, and assembly in plasmodial extracts and development of an improved purification procedure

Actin is readily extracted from plasmodia of Physarum polycephalum by low ionic strength solutions which do not solubilize the plasmodial myosin. The actin in such extracts exists predominantly as a monomer which slowly denatures, apparently via removal of bound nucleotide, and is subsequently proteolyzed. However, the native monomeric actin can be induced to assemble into polymeric arrays under appropriate solvent conditions. Actin assembly is dependent on the addition of ATP and is a function of KCl and CaCl2 concentrations. These observations have allowed the development of an improved actin purification scheme which is simple, rapid, and efficient, yielding approximately 60 mg of protein from 100 g of plasmodium. The actin thus obtained is pure, stable, and comparable to that obtained by previously described procedlres. Furthermore, the observations suggest that actin polymers may be metastably assembled in vivo and raise the possibility that actin assembly, and plasmodial movements, could be regulated via alterations in intracellular concentrations of nucleotide and/or divalent cation.