RNA metabolism during cytodifferentiation in the cellular slime mold Polysphondelium pallidum

Regulation of synthesis of cell-specific ribosomal proteins during differentiation of Dictyostelium discoideum

Proteins of ribosomes from various stages of development in Dictyostelium discoideum were analyzed by two-dimensional polyacrylamide gel electrophoresis. Significant changes in protein composition were observed; the data demonstrate that cell differentiation in a eukaryotic system is accompanied by ribosome heterogeneity. Both qualitative and quantitative differences were noted for 12 unique ribosomal proteins between the vegetative amoebae and spores (differentiated cells). Two proteins were specific to ribosomes of amoebae, and three were specific to spores. The others were common to both cells but showed characteristic stoichiometric changes. The appearance and quantitative changes of these proteins were associated with specific stages of cell differentiation and were evident only during the aggregation phase; however, further changes continued through construction of fruiting bodies. As functional mRNAs for all 12 proteins were present in both amoebae and spores, both transcriptional and translational mechanisms apparently regulate the synthesis of the various developmentally controlled ribosomal proteins in the two cell types.

Repression of rRNA synthesis induced by disaggregation in Dictyostelium discoideum

Cells disaggregated from the slugs of Dictyostelium discoideum continued to incorporate [3H]uridine or [3H]uracil, though at a lower rate than interphase amoebae. Disaggregation brought about a marked increase in the proportion of labeled poly(A)-containing RNA to labeled total RNA. The proportion reached the maximum in disaggregated cells after 1 h of incubation, but remained high even after 5 h of incubation. Reconstruction of slugs from the disaggregated cells did not bring about a decrease but a further increase in the proportion. The real proportion estimated from the minimum period of labeling reached almost 100% in the disaggregated cells, in contrast to about 30% in the interphase amoebae. The increase in the proportion is attributed to the disaggregation-induced repression of rRNA synthesis, which was supported by sucrose density gradient analyses of RNA synthesized in the disaggregated cells. The possibility that rRNA synthesis in this organism is regulated by the loss of cell contact is discussed.

Conservation of the 5'-terminal nucleotide sequences of ribosomal 18-S RNA in eukaryotes. Differential evolution of large and small ribosomal RNA

The 5'-terminal nucleotide sequences of ribosomal 18-S and 28-S RNA of seven species of eukaryotes including three mammals, one bird, one amphibian, one echinoderm and one slime mold, were analyzed either by means of terminal phosphorylation of RNA with polynucleotide kinase of by fingerprint analysis of uniformly labeled RNA. The following conclusions were obtained. 1. The 5'-terminal sequences of the 18-S RNA of the mouse, chicken and Dictyostelium discoideum were pUpApCp(Cp,Up)Gp---, suggesting strongly that all the eukaryotes had this same sequence at the 5'-terminus. Preliminary analysis of the 5'-termini of the 18-S RNA from human, rat, Xenopus and sea urchin cells revealed the same pUp(Np)Gp--- type 5'-terminal structure, supporting the above hypothesis. 2. The 5'-terminal sequences of the 28-S RNA of the human, rat, mouse and chicken cells were all pCpGp---, whereas those of the lower animals such as Xenopus, sea urchin and Dictyostelium were different.

Biochemical changes during growth and encystment of the cellular slime mold Polysphondylium pallidum

The growth of the cellular slime mold, Polysphondylium pallidum, was studied on a semidefined medium in shaken suspension. When the medium contained large quantities of particulate material, growth was more rapid and the cellular size and protein content were smaller than when growth occurred on a medium containing less particulate material. The cellular levels of DNA, RNA, and protein; of lysosomal enzymes (acid phosphatase, acid proteinase); and of peroxisomal enzymes (catalase) were assayed during growth and the subsequent stationary phase that led eventually to encystment. Only DNA remained at a constant cellular level. Encystment of exponentially growing cells could also be initiated by washing them and introducing them into a soluble peptone medium. The rate of encystment was proportional to the osmolarity of this medium. The encystment process was followed with respect to the cellular levels of DNA, RNA, protein, carbohydrates, acid phosphatase, acid beta-N-Ac-glucosaminidase, and catalase. The most dramatic change occurred in the cellular cellulose content, which increased by at least an order of magnitude by the time encystment was morphologically complete. It was concluded that the encystment of this slime mold in suspension exhibits a number of biochemical similarities to the development of this and other cellular slime molds on a surface.

RNA synthesis and turnover during density-inhibited growth and encystment of Acanthamoeba castellanii

Alterations in transcription that precede and accompany encystment (E) of suspension grown A. castellanii have been investigated. Comparative studies were performed on cells undergoing spontaneous E in high density stationary phase cultures or after experimental induction of E at low cell densities by deprivation of nutrients in exponential growth. Onset of growth deceleration at high cell densities was accompanied by an increase in the cellular RNA. The maximum RNA content occurred in cells at stationary phase and subsequently declined with the appearance of cysts in the cultures. On the contrary, the RNA content in cells whose growth was immediately terminated by experimental E induction remained at a constant exponential level through 5 h postinduction and then began to decline shortly before the appearance of cysts. The mature cyst formed in stationary phase cultures and after experimental E induction contained an equivalent amount of RNA ( approximately 50% of the exponential value). Comparison of the kinetics of [(3)H]uridine incorporation demonstrated that there was an abrupt reduction in the rate of uridine incorporation into RNA with onset of growth deceleration or after growth termination in experimental E induction. The reduced incorporation of uridine into RNA could not be attributed to to a reduced uptake of the isotope by the cells or an altered capacity of the cells to phosphorylate uridine. Uridine continued to be incorporated into RNA at a reduced rate in cells throughout growth deceleration, in stationary phase, and up to 12 h postexperimental induction. Considered together, these results indicate that a buildup in RNA is not necessary for induction of encystment in acanthamoeba. The accumulated RNA in stationary phase cells appears to be due to the greater reduction in the growth rate than in transcription and the absence of RNA turnover in cells during growth deceleration. Initiation of RNA turnover appears to accompany growth termination and induction of E. The results further demonstrate that the regulation of the rate of transcription is closely coordinated with the control of growth and encystment in acanthamoeba.

Ribonucleic acid synthesis during microcyst formation in Myxococcus xanthus: characterization by deoxyribonucleic acid-ribonucleic acid hybridization

The technique of deoxyribonucleic acid-ribonucleic acid (RNA) hybridization was used to compare the RNA synthesized during vegetative growth and microcyst formation in Myxococcus xanthus. All classes of RNA, including ribosomal RNA, were synthesized during microcyst formation. The results indicate that the ribosomal RNA synthesized during microcyst formation was indistinguishable from that made during vegetative growth. Hybridization competition experiments demonstrated that certain messenger RNA species are synthesized only during vegetative growth, whereas others are synthesized only during microcyst formation. The synthesis of a new species of RNA polymerase does not appear to be responsible for differential transcription during morphogenesis in M. xanthus since the rifampicin sensitivity of transcription was conserved during microcyst formation.

RNA in cytoplasmic and nuclear fractions of cellular slime mold amebas

A method is described for the rapid separation of cellular slime mold (Dictyostelium discoideum) cells into nuclear and cytoplasmic fractions. Sucrose density sedimentation profiles of radioactivity from cells that had been grown for long or short periods in the presence of uridine-(3)H indicate very low levels of cross-contamination between the fractions. The nuclear fraction contains few, if any, ribosomes. In exponentially growing cells, at least 80% of the ribosomes were associated in polysomal complexes. No loss of counts from pre-labeled rRNA was observed during 2 generations (24 hr) of logarithmic growth and, within the polysomal complexes, the distributions of the preformed material and of rRNA synthesized during the 2 generations were identical. In stationary phase cells that had entered the developmental program leading to fruiting body construction, the rRNA turned over rapidly so that by the end of development at least 75% of the ribosomes fabricated during exponential growth had disappeared and had been replaced by new ones synthesized during the morphogenetic sequence. The preformed ribosomes disappeared preferentially from the monosomal contingent; the newly synthesized ribosomes appeared exclusively in the polysomal contingent and did not appear as monosomes in appreciable numbers for at least 6 hr. The possible significance of this wholesale replacement of ribosomes is discussed.

Alternative developmental pathways determined by environmental conditions in the cellular slime mold Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum grows in the soil as a population of independent, uninucleate amoebae. Upon entrance to the stationary phase, the amoebae collect in multicellular aggregates to form organized fruiting bodies composed of spores and stalk cells. Depending upon environmental conditions, the developing aggregate either constructs the fruiting body at the site of aggregation or transforms into a structure that can migrate to a more favorable location. Environmental conditions that favor migration are (i) the accumulation of metabolite(s) produced by the aggregate and (ii) a low ionic strength in the substratum. Conditions that prevent migration or that stop a migrating slug are (i) the presence of buffer and (ii) illumination by overhead light.