Polyphosphate in the life cycle of Physarum polycephalum and its relation to RNA synthesis

Nuclear polyphosphate as a possible source of energy during the sporulation of Physarum polycephalum

31P NMR spectroscopic analysis of the polyphosphate pool in cellular and nuclear extracts of Physarum polycephalum demonstrates that plasmodia and cysts contain inorganic polyphosphates with an average chain length of about 100 phosphates. However, only during sporulation are these high-molecular-weight polyphosphates degraded to a lower molecular weight corresponding to an average chain length of about 10 phosphates. Since polyphosphates are degraded even in the presence of a sufficiently large pool of inorganic phosphate, produced by intracellular injection, we conclude that the degradation of polyphosphates serves in supplying energy for biosynthesis during sporulation rather than in increasing the availability of phosphate.

Stored messenger ribonucleoprotein particles in differentiated sclerotia of Physarum polycephalum

Starvation induces vegetative microplasmodia of Physarum polycephalum to differentiate into translationally-dormant sclerotia. The existence and the biochemical nature of stored mRNA in sclerotia is examined in this report. The sclerotia contain about 50% of the poly (A)-containing RNA [poly(A)+RNA] complement of microplasmodia as determined by [3H]-poly(U) hybridization. The sclerotial poly(A)+RNA sequences are associated with proteins in a ribonucleoprotein complex [poly(A)+mRNP] which sediments more slowly than the polysomes. Sclerotial poly(A)+RNP sediments more rapidly than poly(A)+RNP derived from the polysomes of microplasmodia despite the occurrence of poly(A)+RNA molecules of a similar size in both particles suggesting the existence of differences in protein composition. Isolation of poly(A)+RNP by oligo (dT)-cellulose chromatography and the analysis of its associated proteins by polyacrylamide gel electrophoresis show that sclerotial poly(A)+RNP contains at least 14 major polypeptides, 11 of which are different in electrophoretic mobility from the polypeptides found in polysomal poly(A)+RNP. Three of the sclerotial poly(A)+RNP polypeptides are associated with the poly(A) sequence (18, 46, and 52 x 10(3) mol. wt. components), while the remaining eight are presumably bound to non-poly(A) portions of the poly(A)+RNA. Although distinct from polysomal poly(A)+RNP, the sclerotial poly(A)+RNP is similar in sedimentation behavior and protein composition (with two exceptions) to the microplasmodial free cytoplasmic poly(A)+RNP. The results suggest that dormant sclerotia store mRNA sequences in association with a distinct set of proteins and that these proteins are similar to those associated with the free cytoplasmic poly(A)+RNP of vegetative plasmodia.

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.

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

Fluctuations in the pools of deoxyribo- and ribonucleoside triphosphates have been measured during the synchronous mitotic cycle of the slime mould Physarum polycephalum. The most pronounced fluctuation of the deoxyribonucleoside triphosphates was seen shortly before and after initiation of DNA synthesis. The pools of dTTP, dATP, dCTP and dGTP expanded before initiation of DNA synthesis and decreased again in early S phase. The pools of ribonucleoside triphosphates increased during mitosis and again 1 h after mitosis and 5 h after mitosis.

Condensed phosphates in Lemna minor L. and their relationship to nucleic acids

After previous incubation in a P-free culture medium whole plants of Lemna minor L. were given [(32)P]-orthophosphate in the logarithmic and in the stationary growth phase. The condensed phosphates could be separated from the nucleic acids by fractionation on methylated serum albumin and Kieselgur (MAK). The various inorganic condensed phosphates were eluted before the nucleic acids. The former were dialyzed and separated by two-dimensional thin layer chromatography on a mixture of cellulose and microcrystalline cellulose using an acid and an alkaline solvent in succession. (32)P-radioactivity was present in all linear oligophosphates containing 2 to 7 residues, in cyclic metaphosphates (tri-, tetra-, penta- and hexametaphosphates) and in high-molecular condensed phosphates which remained at the starting point.Among the low-molecular inorganic oligophosphates the trimetaphosphate contained significantly more radioactivity than the other compounds. The labeling rate of (32)P-incorporation depended on the previous depletion of phosphate. In the logarithmic phase maximal labeling occurred after 4 h following a 10 h period of phosphate depletion; in the stationary phase the maximum rate was reached 5 h after the end of a 4.5 h period of phosphate depletion. The results indicate that the phosphate pool was restored first and that the nucleic acids were labeled subsequently. That is to say: in contrast to previous findings by other authors, our results show that net synthesis of inorganic condensed phosphates does takes place during the logarithmic phase.After cultivation of Lemna for 24 h or longer in a P-free medium the DNA incorporated more (32)P than the other nucleic acids during the logarithmic phase. During the stationary phase [(32)P]-orthophosphate incorporation into nucleic acids was markedly lower than during the logarithmic phase.